Best Framework to Develop Apps for Kids?

The Swift Playgrounds app has many examples of playground books that would appeal to kids, and which use a variety of Apple frameworks (SpriteKit, SceneKit, ARKit, UIKit, AVFoundation, Photos, etc.). There are puzzles and games, as well as camera-, photo-, music- and sensor-based Challenges. Looking at these should give you a good idea of which Apple frameworks could be useful for the kind of app you have in mind. In addition to the Challenges, there are related Starting Points in which you can remix code or develop your own projects. An example would be the Sonic Workshop challenge with its corresponding Sonic Create starting point.

I just ran an online class for kids where we took the Lights, Camera, Code playgrounds (after having gone through the Everyone Can Code Puzzles book -- https://books.apple.com/us/book/everyone-can-code-puzzles/id1481279769 -- with the kids) and used their infrastructure to build apps. They were getting ready to submit something for the Swift Student Challenge (but were a bit too young... you needed to be 16+ in Europe to submit.).

It's a great starting point once you've done the Learn to Code playgrounds!