I've not played it but I hear that Magical Kitties is a great RPG for kids, if it's the kids style of course.

Otherwise I'd absolutely recommend just hacking the one-page-rpg Lasers and Feelings. Hacking it is really really easy. You just pick two good buzzwords that oppose eachother and clearly set the tone of the game, ideally a planning word and a doing word but I once ran a hack called Scandal and Virtue that worked really well as it's basically the lawful - chaotic scale. Here are a list of hacks other people have made.