Oh God, do not do this! We had a party with 30 first graders and it was awful. Like, almost dangerous awful. Unless you have at least 10 parents to help, don't do it. At that age, the boys will be knocking everyone around (just playing, but still), the girls will be screeching - oh man, I'm getting flashbacks. There is no way in hell you can control that many kids. Don't do it. But if you do, please post a video
