I bet they weren't limiting the number of kids in there at a time, letting so many in there at once that it was inevitable that somebody would get kicked.
My daughter attended a birthday party when she was in kindergarten that had a bouncy house, and the people who brought it limited the number of children who could bounce inside it at one time, which made a lot more sense than just letting a lot of rowdy big kids crowd in with the smaller kids.
I think that the birthday party where Sam Harris was hurt was for eleven year old triplets, he was eleven years old at the time, and it was a fifteen year old turning somersaults who caused the injury to him.