My 9 and 12 year old have both played a fair bit with Pico-8, and while the music and sprite tools are neat, I think there's a lot that it leaves on the table. The Lua environment is a fun puzzle for seasoned developers to reason about how to make the most of it, but for a novice, stuff like physics, collision detection, and an entity system out of the box is more like the right starting point. You need a lot of boilerplate to get a sidescroller off the ground in Pico-8.