Has anybody had much experience playing DnD or other tabletops with children? I've been toying with the idea of making a fairly straightforward and simplified RPG using Story Cubes and GURPS that kids can get involved with easily and have fun playing. I'm specifically aiming to play with my daughter (8) and my niece (5) on a big family holiday in August, though I see no real reason that this couldn't work with adults as well.
Essentially, the conceit would go along the lines of each player rolling a limited number of story dice to help with character creation and such. I'd ask the players a few simple questions about their powers (for example, are you more of a wizard or more of a warrior?) to get some basic stats stats together (STR, DEX, INT, CON), and then use story dice myself to quickly improvise a short one-shot session.

Does anyone have experience playing with kids, and if so - any pointers? Am I being too ambitious about children's ability to imagine stuff in this way? If so, are there any good systems out there that are good for young people to pick up and get stuck into roleplaying with?