Ugh... it's crazy when kids do this!! An angel all day (haha, maybe not ALL day) but turn into such a terror at pick up.
My handbook states that while parents are dropping off or picking up, THEY are in charge of their children and are expected to enforce MY house rules. The basic ones are stated in my handbook as well.
Having said that, if mom and dad aren't going to, I darn sure will! I have said in other posts; my breaking point was when a 5 1/2 year old KICKED my front door into the wall, because he didn't like what dad said they were having for supper. (He was a real treat, let me tell ya!!)
From then on, I was ON HIM until he was out of my sight. If he was smacking mom or whining and crying, I was the one who ceased the behaviour. "We do NOT hit our mom, or anyone else, and you know that. If you continue, then tomorrow when you come, you will not be playing with (insert favourite toy/activity)." He was old enough that future consequences worked.
The beauty of doing this is that parents who have "given up" see that it is, indeed, possible to reign in this behaviour. Sometimes, that's enough for them to change also!