If you haven't ordered, you walk out of the store. If you have, you ask for to-go containers, pay and leave.
I promise it will make such an impression on them that they will NEVER do it again. You, as a parent, have to be willing to leave, but you only have to do it once.
Oh, and do it quietly. Don't scream, don't yell, don't throw a fit. That's what they're doing. Your turn to be the parent.