I had the same hopes for my kids with PICO-8, but it didn't really pan out— the things they were excited to do (make a platformer!!) all require a ton of boilerplate around movement, gravity, collisions, sprite animation, and so on. The high profile projects on the platform are all super polished and based on endless hacks and tricks that push the boundaries of the system, and it can be hard to get away from those expectations.
GameMaker has gotten pretty professionalized at this point, but back in the day I think it was a good entry point, with decent non-code workflows to get the basics going and a smooth on-ramp to adding behaviour code as needed.
I don't know what fills the hole that used to be platforms like ZZT and MegaZeux, though. Maybe it's Minecraft and Roblox that are the modern outlets for that kind of creativity.