Here's my two cents.
1) The most important thing on YouTube is sound. The audio on "What's inside my Japanese School Bag?" was subpar. People are willing to put up with subpar visuals but not subpar sound. It appears from the videos I watched, you're using the camera's microphone. Those are almost always poor. Camera makers focus more on visual than audio and partly because they know pros and semi-pros will use their own mics. What you need to do is lav mic your kids.
And because you have kids, I would strongly recommend you get a wireless lav mic. You don't want either of them to get excited by something, run after/to it, and yank the camera out of your hands. On the bright side, wireless lav mics will enable you to get crystal sound with them far away from you. Right now you're crowding them so you can pick up their sound. Wireless lav mics will enable you to have a more comfortable distance from them. In fact, you can have them climb rocks, zoom in on them, and viewers will hear them clearly. Now you do pay for distance. The more you pay, the further away they can be. Just another variable to take into consideration when buying a set-up.
Another thing you need to keep in mind is weight. What is insignificant to you might not be to your kids. The kid I would use as the weight gauge is your son since he is younger and thus smaller than his sister. This will require a good deal of research. You need to get one that BOTH of them are comfortable wearing and ideally forget that they're wearing. Again, because they're kids, size also matters. Look to get as small of a wireless transmitter as possible. This you will clip to their waistline and, again, weight might play a factor with their clothes. You don't want the weight to be constantly pulling down their pants/skirts. I would not recommend duct-taping the transmitter to the kids. Kids usually get weirded out by that. You might need to rig up a separate belt for them. Belt comfort then becomes crucial with kids. Don't think anything like "Well, they'll get used to it." They likely won't and instead they may get fixated on it and constantly complain about it.
And, this probably goes without saying but just in case, you run the mic's wire under their clothes to the wireless transmitter and not over their clothes. Some kids take a little time to get used to this as this isn't what they're normally used to wearing. If they say, "It's sticky." simply get some baby powder and dust the wire and their skin that the wire touches with it. If they say the transmitter is hot, you can put one of those large Band-Aids as a cushion and insulator. If it is still too hot, you need a different transmitter. Another thing you might want to consider doing is putting the transmitter on the front or side of their pants/skirt when you go sightseeing as then it will actually be less visible since you're following your kids then.
Go to an electronics store and have the kids try out the possible set-ups (clip, wire (to transmitter), and wireless transmitter) like they do shoes. Have them walk and run around, jump up and down, twist right and left, bend forward and backward (backwards is usually when they'll notice the transmitter the most since it will be attached to the back of their waistline), etc. I don't know what kind of money you can spend or what is available, but you can usually get clipped mics and their wires in different colors so you can match them to what your kids are wearing.
Another issue can be frequencies that the wireless lav mic will use. You appear to be in a major city (Tokyo?) and that means available frequencies can be an issue. If you have a halfway decent electronics store, they'll have someone knowledgeable about this issue, how it plays out locally, and what, if anything, you need to do about it.
Having said the above, now I'm going to make you groan. You have two kids and they seem to be in most videos, though it does seem like your daughter Maya is the channel's main focus. Unfortunately, if you want your son Alex to be part of the show, you're now dealing with two audio feeds. How to deal with them can be done in many different ways. Ranging from quick-and-dirty to long drawn out and labor intense. BUT, again, sound quality is crucial for YouTube so the more you're willing to do for it, the better. The quick-and-dirty way is to simply jack the wireless feed directly into your camera BUT this gives you very little control over the audio and if you do try to control it, it is actually the most labor intense way to do it. Instead, I would recommend you record their audio as two separate audio tracks and add them in post production. HOWEVER, keep the camera's microphone on! If the lav mics fail, you will at least have that.
Another thing I would recommend is that you get a set-up that enables you to put a ear bud into one of your ears so you can monitor both audio feeds. Wires can get unplugged. Batteries die. Wind can destroy reception. If you are not actively monitoring the audio, you won't learn the mess you have until you're in post-production and then your neighbors will call the police because they will think you're screaming because someone is trying to kill you. Save the pain and live monitor audio.
Sorry, I don't have recommendations for wireless lav mics. There's just too many variables to take into consider, including what your budget is. Visit a number of stores. Get input from all. Then make your decision ... then later regret your decision ... then learn from the School of Hard Knocks and buy a better set-up ... then later regret your decision, whine about how much money you spent for the piece of cr*p, and buy a better set-up ... then later regret your decision and ... lather, rinse, repeat. Sorry. That's just the nature of audio. You learn by doing and the tuition sucks.
2) Get a camera stabilizer (a.k.a. gimbal). Your visuals are shaking too much, especially noticeable when you zoom in on Maya, as was clear in the video "JAPAN FOOD- Udon". I can almost make out the rate of your heartbeat. And panning between your children was WAY too abrupt. [Just FYI, sound was subpar on that video too.] I would strongly recommend you check out the gimbal reviews by Dave Dugdale (channel by same name) and Film Riot. As you're out and about in public, don't DIY this. Get a nice-looking MOTORIZED one made by a factory. Get it one-handed and compact. Yes, two-handed stabilizers are FAR better than one-handers, but you'll need your other hand ... if only to wave at your kids to come back from the cliff's edge. Seriously, you'll need the free hand to direct your kids, keep the audio feed ear bud in your ear, hold onto a railing while filming over the edge, etc. Just know that ANYTHING will be better than what you're doing now. Go to camera stores and test them out. Keep an eye on their weight. If two are comparable, go with the lighter one. If possible, RENT the one you're thinking of buying and use it for a weekend to test it out. If there are two that are truly in the running, rent both and test and compare. Once you buy one....
Camera stabilizers take time to get used to. Just go out and film by yourself. Forget the kids. Walk about town. You need to get to the stage where the stabilizer is a natural extension of your arm. Find out how long you can hold the entire set-up in one hand. This is where weight comes into play. Practice switching from one hand to the other to make it smooth. Learn how long the gimbal's battery lasts. Dave Dugdale takes about six months to get used to a new gimbal and he's a professional photographer with decades of experience. Put in the time. Master it.
3) Buy a pair of LED lights whose support structure your camera sits on and always use them. Have them as far apart from each other as possible and that is still socially acceptable. As you're in Japan, I have no clue what is socially acceptable there. What you want to do is kill the shadows on your kids' faces. Naturally get light-weight lights. Don't have them run off of the camera's battery but get and wear a battery pack. Yes, you can have the pack power your camera and it then power your LED lights. Don't have the lighting structure support the battery pack as that adds unwanted weight. Instead clip it onto your belt. Go to a camera shop and check out what they have. This is another one you shouldn't DIY as you're out filming in public a lot.
And as you're using LED lights, you can use color and diffuse gels to enrich your footage. LED lights have very very little heat and will not melt the gels. Learn the different settings on your LED lights and what they're good for. No single setting is right for all conditions. Remember that free hand you have because you got a one-handed gimbal? Use that hand to adjust the lighting on the fly. If you can find them for your LED lights, get snap-on gels that you can whip on and off with your one free hand. Both adjusting the setting and manhandling the gels will take practice. But, eventually, you'll be adjusting the lighting and gels without even thinking about it. Sort of like riding a bicycle.
4) If you want more of an international audience, you will need to do list episodes. Sorry, they get the clicks. I looked over your entire video library and didn't see a single list episode. Have Maya narrate them. Specialize in just Japan lists. Fortunately for you, Japan is the freakiest nation in the world. It even revels in its freakiness.
To get you going on list episodes, check out all the ones that have already been done on Japan. There are a LOT of them. Watch them all. Think how you would do them differently. At least different ranking. Removing some too-adult topics. Lots of list episodes copy each other or are very similar. Write down all the lists other people have done. First, do them. Cut your teeth on them. Yes, you're copying them. Yes, that's not original. So what? Your daughter Maya narrating them will make them different. Second, go out into the city and show where the stuff is. Don't show a still photograph but have Maya talk in front of where they are in Japan. Third, once you've copied all those list episodes, come up with your own. Look around you. The VAST majority of YouTubers who have done a list episode about Japan don't actually live there or have ever even visited it. Use that edge to your advantage.
5) Develop a fan base. Call it something "cool" (as defined by Maya) like "Maya's Musketeers". Have Maya address them directly in your videos. Call upon them. Ask them questions for them to answer in the comment section. Do meet-ups with those in Japan and make episodes out of these meet-ups. If a fan sends Maya and/or Alex a present, do an unwrapping episode of it. If a fan sends in a drawing of Maya and/or Alex, have the recipient thank them and then put it up on a special wall for just fan art. If you're going to a YouTube or some other kind of convention, have Maya tell where, when, and if anyone is interested in meeting up to leave a comment and one of her parents will get into contact with them and possibly set up a meeting. Limit meet-ups to ONLY kids and families with kids. A meeting that will become an episode. If a group of them want to meet Maya and/or Alex, Maya could say they could then do X activity at the convention. The "X activity" being some activity special to that convention.
6) Start up Maya Con ... or Maya & Alex Con ... or Chottomattenet Con ... or Maya's Musketeers' Con in the city where you live. This is just an annual fan meet-up with silly aspirations of becoming Japan's VidCon. Keep the first one light and informal. Let it organically grow. Ask your viewers for ideas and volunteers. Have it have a kid focus. Invite Japanese YouTubers to it. You're in Japan so think freaky. The more kid-friendly freaky your con is, the more media coverage it will get. Talk to local restaurants about holding it at their place with the price of admissions (for at least the first year) going to pay for a kid-friendly buffet at the restaurant.
7) Start up a Patreon page and have third of its money go into a college fund for Maya and Alex, a third saved up for a family vacation that you'll then make a bunch of episodes out of, and third for you producing this channel. For that last third, it is best to tell what new equipment you'll buy or service you'll employ that will enrich the show. Think up interesting incentives that cost nothing to produce. A special thank-you video for just them. A shout-out to them and even their channel. An original piece of artwork by Maya and/or Alex. And for the big donors who are in Japan, dinner with the whole family. Visit the Patreon pages of family vlogs and get incentive ideas from them.
8) Contact schools in Japan, tell them about your channel, that you live in Japan, and ask if they would like Maya and/or Alex to come by and visit their same-age classes. Volunteer Maya and/or Alex to answer questions from the students about their home country. Suggest that the entire class give Maya and/or Alex a tour of their school. Saying this is socially appropriate in Japan, tell them that you will bring beautifully-wrapped gifts for the school's superintendent, principal, and all the teachers of the visited grade and that the gift presentation will be made part of the video. If they give a gift in return (which I think is required by Japanese culture, right?), tell that you will do a special gift-unwrapping episode of it once you get home. As I believe opening a gift in front of a gift-giver in Japan goes against Japanese etiquette, that should hopefully be more than acceptable. Every school you visit will add hundreds, if not thousands of subscribers to your channel.
9) Contact Japanese celebrities (or more likely their publicists) and ask if they would like to do an episode where they at least meet your daughter and son. Tell of how many subscribers you have, what percentage are Japanese, and suggest activities to do, such as helping Maya do one of her list episodes. If they are actors and actresses, ask for them to give Maya and/or Alex a tour of their TV or movie sets. If they're animation producers, ask if Maya and/or Alex can do a bit of guest voice acting (such as barking for a dog character) for one of their episodes and you filming them from the control room while they do their voice acting in the sound studio. If they're a politician, suggest they take Maya and/or Alex onto the legislative floor so they can sit in their legislative chair as you film them from the balcony. If they're a game show host and their show uses models to show off prizes, ask if Maya and/or Alex can help their models for one of their episodes. If they're a news anchor, suggest ... you get the idea. ALWAYS suggest a specific activity that enables the celebrity to show off and/or to be a big celebrity. After you've come to them, send them a thank-you card and in it give them an open invitation to do a surprise drop-by of your house. Who they're surprising is your kids. You know they're coming and are ready for the stop-by. The simplest and easiest thing the celebrities can do on these surprise visits is for them to ask your kids for a tour of your house. After they have done a surprise visit of your house, send them a thank-you card and suggest that your kids then do a "surprise" visit of them. For your kids, they think they are surprising them, but they're not. They just have to think they are for it to make a great episode. The celebrities will then dress for the part. Some humorously will be in their lounge jacket or with their hair in rollers. Many celebrities will love to ham it up. After you "surprise" them at their house, send them a thank-you card and then ... think of something else fun for them to do with your kids. Lather, rinse, repeat.
This deserves its own paragraph. Start working to get Maya and/or Alex to meet with the Emperor and/or Empress of Japan. Yes, yes, I know this will likely be difficult, if not impossible. But what is the harm in trying? What's the worst that could happen? "Thank you, but I'm afraid the Emperor's duties preclude such a..." Then again, as he's a kid, maybe you should shoot for Prince Hisahito. The prince could take Maya and/or Alex around for a tour of the imperial palace.
As for American, British, Australian, Indian (as in the country India), and other English-speaking celebrities, when you learn of one that is coming to Japan, contact them (likely their publicist) and see if they would like to do an episode with your children. As for what to do in the episode, I would suggest you suggest they let your kids give them a tour of something in Japan. Like your Maya's and/or Alex's classroom and school while school is in session. Or Maya's and/or Alex's favorite restaurant. Or some tourist attraction where filming by the public is allowed.
10) Have Maya speak out for a charity and/or social cause. Does Maya have a favorite charity? If so, that's it. If she doesn't, have her spend time finding one that she REALLY likes and would like to help. Ideally one that is national and not local or international. Japan is your niche. Embrace it. Don't be concerned about how many members the charity has. It is FAR more important that Maya really believes in their cause and wants to help them. Don't worry that the charity might already have a celebrity spokesperson. I'm willing to bet you that that spokesperson isn't a kid. But before promoting the charity, contact it and see if they would like your help. If they don't, don't argue and pick another charity. If one is receptive, talk about what Maya could do to help raise awareness for their charity. The charity's PR staff should have lots of suggestions. Work with them. Educate them on what Maya can and cannot do. What you can and cannot do. Stress that what she does should be something she can do for an episode. Say if the two of you can come up with enough great ideas, this could be another weekly episode that Maya could do. Almost all charities will then promote her and your channel to their donors and that will help bring in more subscribers.
11) Get your subscriber's email addresses with a special monthly unlisted video. At the end of each of your videos, offer to email them links to a special monthly unlisted video if they give you their email address. Right now, YouTube doesn't share this with YouTubers and it is good to have. It gives you a way of directly communicating with your viewers for special announcements and makes you more independent of YouTube. You get too upset with YouTube and you can move you subscribers to another platform (i.e., Vessel). Do NOT send any SEPARATE special announcement but use the monthly link-giving email for them. As for what that special monthly unlisted video can be, it could be a monthly Q&A ... fashion show by Maya ... Maya and/or Alex doing a cover of a Top 10 song ... whatever you can think of that is fun to do, fun to watch, and can be done over and over and over again. Additionally, you can send out links to unlisted videos for just your fan base that might not be right for your public channel. What wouldn't be right for your public channel, I have no idea but that's for you to decide.
12) Redo your intro video. First have Maya talk about what kind of things she talks about on your family's channel ... then who you guys are (people subscribe to people, not channel) ... then tell your release schedule ... and then do ONE call-to-action and that is for them to subscribe. Don't monetize your intro video. The intro video is an ad for your channel. Don't have another's ad hurt your ad.
I know how much detail you tell about your family is and should be a delicate issue, but the more you can tell, the more people will connect to you. Where were you guys before you came to Japan? Why are you in Japan? Those two questions should be addressed in your intro video.
13) Increase your release schedule. The more frequent, the better. You are trying to develop a relationship with your viewers. That takes being part of their daily life as much as possible. You can do what you do now and simply add other kinds of episodes, such as those I've previously mentioned.
14) Make sure Maya always does the following at the end of each episode: A) ask your viewers a question for them to post their answer to in the comment section, B) tells your release schedule at the end of very video, C) asks for new viewers to subscribe, and D) ask for their email address for a monthly unlisted episode. If you have different types of weekly episodes, tell their different release schedules. "On Mondays, we release our family vlog and on Fridays we release our list episodes."
15) If you are monetized, you can use the scheduler to regularly release videos. Always always ALWAYS build up a backlog of videos sitting in your scheduler. Do this not just for vacation time but in case you get sick, your computer crashes, your internet connection goes down, you get writer's block, kids become sick, kids go on strike for a higher allowance, etc. It also gives you breathing space. If you release three episodes, six videos in your scheduler represents two weeks of episodes before you have to produce another video.
16) Work on your thumbnails and video titles. Do a YouTube search on how to do thumbnails and video titles. There's a lot of good videos out there about both topics.
17) Subscribe to Tim Schmoyer's "Video Creators" and mine its archive. You can and should spend days in its archive.
18) Make up a flyer. Use yellow paper to draw the eye. Have tear-off tabs at its bottom that has your channel's name and its YouTube URL. Have one side be in English and the other in Japanese. Post it everywhere you can within reasonable driving distance. By "reasonable" I mean as far as you are willing go to promote your channel. Laundry mats, bus stops, supermarket bulletin boards, and telephone poles at intersections are good places to post, BUT do check out what's legal in Japan and in your specific city. Get a map of your city and mark on it where you've posted them. Once a week, revisit those locations and replace missing, all tear-off tabs gone, torn, etc. posters. Always keep a box of these flyers in your car so when you travel, you have them right there with you and you can post them wherever you go. If you go on a trip, put a stack of posters into one of your suitcases and take a half day and post everywhere there.
19) Email your local media (newspapers, radio stations, and TV stations) BOTH in Japan and your hometown by sending them a press release about your family and its channel. Stress that you're locals. Newspapers, radio stations, and TV stations are ALWAYS on the look out for "local color".
20) At your daughter's school, encourage your daughter to start a YouTuber Club. Have her talk to her teachers about creating it. Get a teacher to be its sponsor. Ideally a teacher who knows how to work a camera, lighting, etc. Your best bet is her art teacher! Offer to help run the club or even be the one who runs it. The key to the club is the adult who runs it. They need to be excited about doing it. They can also invite local video production companies and TV stations to come and speak to the club. Don't just invite news anchors and radio talk show hosts, but separately invite camera operators, sound technicians, producers, editors, etc. They have a HUGE wealth of knowledge that the club can mine.
Needless to say, be inclusive. The club must be open to all grades in her school and all students are welcome. Don't let it become a clique. Stress that you don't have to have a YouTube channel to be part of the YouTube club. They can just help others (like your daughter) make their videos. You can always use another person in the background at the very least. The more who are in the videos, the better. Meet weekly. Every meeting, show all the videos that all the members did over the last week. Applaud and comment. Encourage. Help each other. Collaborate with each other. Appear in each other's videos. Do skits. Have a group discussion video. Make cookies together. Brainstorm, brainstorm, brainstorm. Most importantly of all, have fun.
21) Lastly but most importantly, collaborate, collaborate, and collaborate. You're a relatively small channel still but everyone has to start from somewhere. Your best bet is to contact local YouTube channels in Japan. As you grow, contact larger and larger channels that match your new size. Always propose an activity for the collaboration and never just that you want to do one. If the YouTuber is Japanese (whether they're a kid or an adult), a simple collaboration video can be them simply showing your daughter and son something that would be interesting to kids. Or they can do a list episode with Maya and you suggesting possible lists you're currently have in development. Have them and your kids (or at least Maya) interact on camera as they tell the list. After you release a collaboration video, send links to it to other local YouTubers to show them how you do collaborations and invite them again to do one with you. Go to ALL YouTube conventions that you can afford to go to. Find out who's going and suggest you meet up for at least a meet-and-greet with them. Doing it while you're at a friendly restaurant gives it a good backdrop. Don't worry about the background noise.
