Under three is free at a buffet, family style meal, or pre-plated meal. However, at any restaurant where you order form a menu, you will be charged for what you order regardless of the child's age.
In other words, when it was our 8 month old, she just ate a few bites from our plates (a few pieces of pasta, whatever) and there was no charge, but when it was our 2 year old, we would order her meals from the children's menu (chicken nuggets, mac & cheese) and we would be charged for these items.
When you make the PS, under three is considered an infant and they will ask you if you need a highchair.
Hope that helps!