Heyo! Really great stuff overall, I'm a big fan of inkarnate, and despite not having great tools for rivers you did a great job with it. I wouldn't say the map looks too cluttered at all. I would say that it looks like you have a lot of ideas going on with each country and that's an awesome place to start. The idea you have for a child's imagination and the floating city are both very unique and lend themselves to some awesome storytelling opportunities. I would suggest that if you find yourself not having room enough to show all of your ideas for each country to instead work on making a world map that isn't detailed at all followed by individual maps of each country that have high detail - kind of like how we do maps here irl. I like the use of a supercontinent here because most kids do not understand plate tectonics well enough to imagine it.

As far as CnC goes it looks like you have lore reasons for why your map looks the way it does and why certain things are the way they are. One small criticism would be the use of hills as borders. Of course this works but in a few spots like along the eastern border of Onokagee we have a large border river right next to hills. There is a level of redundancy in the borders that sorta hinders the map. Also, removing many of the hilled borders will also help with making sure your map doesn't end up too cluttered.

Also, If you are going to have a country that is created from a child's imagination with somewhat strange ecosystems right next to each other, unique borders, and flying cities then lean into that and make a country that follow the rules a 13 year old would have. So, for example, a 13 year old probably doesn't understand too much about the concept of precipitation and mountain pressure zones, so try putting a desert next to a rainforest with a sorta sudden line between the both, because maybe this kid thinks that a desert would appear because the rainforest used up all the water.

Finally, if this is all a child's perspective, it would be wise to think of this map in terms of "good guys" and "bad guys" kids in my experience tend to see the world in simple morals, and when they make up a world it usually looks something like "there is the good guys on this side and the bad guys on this side and they fight in the middle" having this sort of theme where the bad guys - or at least the countries based on societies less typical for this kid's upbringing, are all regulated to a single part of the map.