Just understand that playing a game with kids that young means throwing out the rules as soon as you start. So you need something with absolutely minimal rules (or you're wasting you time) and a story that's easy for them to latch on to.

I love using Honey Heist with kids. It's basically Lasers & Feelings, but you're a bear disguised as a human trying to steal honey from humans. Kids can really take that premise and run with. If you run multiple sessions do it picaresquely, where each session is a self-contained episode.