for ticket sales, Disney actually does not use the designations "adult" and "child." They use "ages 10 and up" and "ages 3-9." They use these same designations for purchase of the dining plan, for paying the lower price at fixed price restaurants, and the ages 3-9 are on the child menus at a la carte restaurants.
In practice at the restaurants, they usually don't stick to those ages at a la carte, but they do for fixed price and dining plan.