I'd vote for D&D 4e. The system is simple if compared to others more complete systems (3.x, Storyteller), and the lack of depper simulationist rules won't hurt it too much. The teddy's are supposed to go back to the kid's confort during the day, so they don't need to "spend the night" and full heal the next "night". And lets your heroes go all hollywoodian on monsters with their fancy powers.