I would just tell the parents what time the party starts and what time it ends. Guests don't get to choose the begin and end times. The hosts do. Just say to the parents, "I'm sorry. We have some other plans too. We won't be able to accomodate guests until XXXX time, and everybody needs to be picked up by XXXX time. I really hope you daughter can still make it because we are going to have lots of cake and ice cream." No special accomodations are necessary. If the kid can be there during the specific times, fine. If not, that's fine too. If she can still make it, don't worry about the food. She'll either eat or be hungry. Too bad, too sad. It sounds like these people treat others like doormats. The kid should either be able to attend on your terms or send her regrets and not come. It's no big loss if she doesn't make it.