There are no restrictions preventing adults from ordering a child's meal. For example, at the (now closed) Concourse Steakhouse my wife would routinely order the child's meal that included a 4oz filet and never had any problems. It is just that the child meals tend to run toward hot dogs, chicken nuggets and mac & cheese as well as being small portions.
There is no such thing as a "kids buffet" or even an "adult buffet" at Disney. They have buffets with one section lower to the floor which has on it items that kids tend to enjoy (such as pizza, hot dogs, chicken strips, mac & cheese, etc.). Any person at the buffet (including children less than age three who are free) can get anything from any part of the buffet. So any buffet or othe All You Can Eat is strictly age-based.