Pizza Hut are offering Kids Eat for free all summer. My wife went with my son today and both had the buffet. She did not pay a penny for him, he had the kids buffet and a kids free refill drink. As long as it is kids meals you order they are free.
It should be under 12 years but mine are 12 and 14, they never ask the age and most of the time assume they are under 12. The regular kids meals would not fill my boys up but it is well worth it for the buffet as they can eat/drink as much as they want.