I played a little D&D with all three of my kids...ages 5, 11, and 13...this weekend. I hope to post about this in more detail at some point, but right now I want to link to this post where a fellow outlines a very simple RPG setup he did for his six-year-old; it contains lots of good idea as far as framing the game in terms a young child might grasp:
That guy thought about things more carefully than I have. We managed to get to a short actual play session after two short character-making sessions.
I'd like to make it very clear that though I think about games a lot and talk about them a lot, I almost never actually play. Somehow friends keep getting the impression that I've gotten actual play going. These sessions with my kids are the first in several years.
Ooh, hey, looks like Wizards of the Coast has a free game/adventure all-in-one thing, in downloadable PDF form. You can find that here: http://www.wizards.com/DnD/Article.aspx?x=dnd/4dnd/monsterslayers