A party of 8 is still a party of 8. Unless your children are the most angelic and well-behaved beings ever born, the servers had 8 (or more) plates, glasses, silverware, etc to clean up, not to mention the assorted detritus that springs up around large parties. Maybe more, since small children are not known for being epicurean diners.

Think of that extra 15% as a lesson on always checking your bill and the small print.