I think it might be an age of kid thing and how many kids are invited to the party, too. When kids turn about 5 or 6, parents aren't staying at parties and the kids want to play. They often have their whole class at the party, plus their baseball team, neighbors, etc (no parents). At a roller skating party or gymnastics party, the kids only have about an hour to skate and play before it's time for pizza and cake. Then, the party's over.