I agree with assigning tasks. At the end of the meal, you could say something like, "Mom & Dad, why don't you go rest while Sis, the kids and I clean everything up for you. After all-you cooked, so we should clean!" Then before anyone has a chance to object start giving out the assignments, "Kids, you clear the table. Sis, if you'll package up the food I'll start washing dishes." And so on.
I can't believe anyone would possibly be rude enough to object/refuse. If they do, then they don't deserve to be invited next time!
I had the opposite problem yesterday. My brother and SIL hosted everyone at their home. SIL wouldn't let anyone bring anything to contribute to the meal-she said we are her guests and she would take care of us (and I must add that it was delicious! SIL is a wonderful cook). After the meal I tried to help with the clean up, but she shooed me out of the kitchen. She only allowed her older daughter (25) to help. Granted, she was the hostess and her kitchen is pretty small, but I felt guilty hanging out with the family in the living room while she was working so hard in the kitchen!