May be time to go the other way and treat the crying as unacceptable behaviour and send her timeout or whatever you do in your home. Call her on it telling her it is ok to miss your parents but they are not here to see how angry (don't say upset) you are so save the tears for at home. Let her know it is unacceptable behaviour and she has the choice to stop or take it to bed or whatever you do.

She is old enough to know that every action has a consequence be it pleasant or otherwise. Make sure the parents are being very quick and matter of fact and no oh you poor dear type behaviour at drop off or pick up that could be feeding into the behaviour. Also remind them about trying to make it up to her with undivided attention in the evening - again will be contributing to it continuing.