Granted, my son is only 7 months (SOB) but I remember being allowed to stay at my friends houses that I went to the same school with. One day is not going to ruin the child. We would have pizza and then go to bed at a later time than usual, but not late - probably 10 - 11. I was up that late at home anyway.
They won't necessarialy remember what they learned in school that day, but they will remember that you let them have fun....