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?

I've run a number of one shots before based off GURPS Lite, and I've come to like the system and its versatility. So, I've taken the next step and drafted a couple interested players from the one shots into a campaign.

Essentially, it's a 150 point campaign in the Infinity Patrol setting, based one of the alternate timelines (Gernsback) developing world jumping tech, and the PCs being one of the first teams assembled from said world to scout out and investigate other timelines. The overarching plot is going to be them defending their world from incursions from Centrum and Homeline and building alliances with other parachronically enabled third parties, like Merlin. The main plot hook is going to be a series of kidnappings, by parties including the Homeline Mafia, Reich 5 Nazis, and Centrum. Because of the nature of their travel, all Quanta are accessible by the party, but not as easily as by projector/conveyor. The worlds they're going to be visiting run the gamut from high fantasy to hard science fiction.

Now that background info's out of the way, basically what I'm asking is: are there any interesting items, characters, or plot beats you've used or seen used in tabletop games that could fit well into this kind of story and setting? I want to put some originality into the worlds they visit so they all feel alive and memorable, and side quests and artifacts are a great way to do that. By no means am I interested only in GURPS stuff, the nature of the system makes it pretty easy to graft in stuff from others.