I would have disciplined them and told their parents at pick up time. I don't think I would have called the parents to pick up right away though. Part of my reasoning for that is because IMO it shows the parents I am not capable of supervising the group. I am not saying this is the case for you, it is just how I would feel and how I feel my group of parents would take the situation.

Depending on the severity of an incident like this, I would tell the parents that if it happened again I would possibly be calling for an immediate pick up and if this behaviour continued than I would have no choice but to terminate.

Kids hurt each other all the time with toys, books, hands and unfortunately branches. I wouldn't terminate after a one time incident but if after an agreed set amount of time, the parents and I couldn't get their child's inappropriate behaviour under control than I would terminate.