Playing these types of games with kids can be hilarious fun.
I played a simple d20 RPG with my 10yr and 13yr old cousins. I told them they were spies and they had to infiltrate this missile base. Twenty minutes later, one was distracting the guards with a robotic monkey while the other snuck in and disabled the base commander by sending laughing gas through the air vents!
Who would've though of that?