I agree with a pp that said to sit the child down and explain what YOU expect from them at pickup and dropoff time. If Mom and Dad can't maintain control at the door then do it yourself.
I've done this before with several children. I tell the child in advance how I expect them to behave and the consequences of choosing not to behave (usually timeout). This can include yelling, hitting, not getting dressed, throwing themselves on the floor, running away etc etc. I also tell Mom and Dad that little Suzy and I are working on behavior at the door so if they could follow my lead that would be great. Then I take over and little Suzy learns to behave at the door very quickly. Mom and Dad are usually so grateful to have a "plan" that they figure out what to do themselves within a few days.
I have no problem AT ALL telling a child what to do when they're still in my home or on my property.