If you are looking for general interactivity with few constraints, Pluto.jl might be useful.
It displays arbitrary graphics, and if those graphics depend on parameters defined in other cells, the graphics will get updated immediately when their params change. You can also do animations by having variables that are bound to time.
On its own it is not geared towards kids though. But its reactivity probably does half of what you are looking for, and its generality should allow you to use it with whichever libraries you want. Using it to play with https://github.com/GiovineItalia/Compose.jl graphics might be accessible and fun.