I found the good old rpg maker quite good when I was at that age myself. It's not that powerful compared to the big name engines but for a plain game it is more than enough. And it focuses more on working with the engine rather than writing code.
You have to take as well into consideration how long it takes for your child to see a result. While I was construction some nice battle logic, my friend was happy to smash a big fireball on a tree in a corner. The faster your child can see a result on what he was working on, the more motivated he will get. Don't over complicate things and just let him/ her play with it.