Hmmm...the restaurant I work at charges 18% for parties of 8 or more...ADULTS. I wouldn't add gratuity to your party, since it was mostly children. And I work at a full-service sit down restaurant, and I agree that you don't get that kind of service at Pizza Hut!
You SHOULD have checked your bill before adding 15% yourself (which is appropriate by the way, and shows that you aren't a cheapskate), but with all those kids...well sometimes you forget.

I think you should have called the store manager right away when you noticed this. He would have been able to fix it. Unfortuanately, I'm sure by this time the server has already claimed the tip on their taxes, etc., so I'd hate to see that money be taken back from them. I hope that if you get refunded the store sucks it up and pays you back from their own profit.And now you have learned to always check your bill before you hand over your credit card!