I've been playing Donjon with my 9 year old son. It's my first try at Donjon and his first serious try at an RPG. We've been playing just the two of us. It feels like a really good intro to RPGs for him:
- When he asked for wacky stuff in character generation (like the teleport gun he wanted), instead of reflexively saying "no, that doesn't fit the world i'm building', the Donjon rules let me just say 'okay, let's figure out how to limit that.' He had in mind a gun that sends opponents away; I limited it to one use per scene.
- He loves the fact-stating capability
- In general I don't have to tell him he can't do something...we just have to figure out how to do it. Successes breed success for all.

We HAVE found it to be awfully crunchy. I wrote a short python script to roll dice for us...it sorts the results so that successes are easy to judge. Since it's just the two of us, we play using a laptop and I take notes as we go along; this works pretty well. Not sure how it would scale as we add people, but he plans to add his sister to the game soon. He's decided she should enter the game as a pet, so he's using a success to notice a pet of one of his goblin opponents...