How strange is it that some kids just get buck-wild at pick up time??
It sounds like you need to become "the Enforcer" now. Do not allow this to happen, as it is disruptive to you and the other kids, not to mention teaching the other kids some bad habits.
My contract states that once a parent arrives, THEY are in charge of thier children, but MY rules are to be enforced. If this does not happen, I will step in.
I used to have a school aged child who was ridiculously spoiled, and would become a whiny little devil child when mom or dad arrived. I don't know if it was embarrassment or reeeeaallllly laid back parenting, but the things he said and did would NOT be tolerated from my kids! The day he kicked my front door because he didn't like what they were having for supper, I was DONE. From that point, I would nip that behaviour in the bud whether parents were there or not.
Maybe it's time to send out a newsletter with drop off and pick up expectations? Remind parents that you are looking after other children, so do not have the ability to spend more than a couple of minutes at the door during these times. Any important issues be discussed at a time more convenient for everyone. When mom/dad arrive, it is imperative that the children understand that it is home time, and that they will come back the next day, etc.
Sometimes I think we as providers forget that people treat us, our home, our schedule, etc exactly how we allow them to. We need to remember that (to a large extent) WE are making the rules here!!