It would be unfair for the children to pay for their parents' rudeness. Could you have a few extras put to the side that you could quickly make up if need be? "Oh, lucky I kept some left overs just in case, otherwise you would have missed out!" *glares at parents* It makes more work for you on the day, is there someone who could help you?

Same with the food, could you have something quick and easy to prepare that you could maybe chuck in the oven if it comes to that?

Or you could hire a bouncer to stand at the door and not let anyone in who isn't on the guestlist? lol