We do a lot of sharing at WDW! We have two pre-teen boys and a pre-schooler, and I routinely share with one of the boys, or they share with each other. If I'm not sharing with them, I usually order a salad and appetizer - otherwise, it's way too much food.
Besides buffets, the other places you need to either pay or use a dining credit for everyone are the family style restaurants, like Garden Grill, Liberty Tree or Ohana. These are not buffets, but they bring family style meals to the table for everyone to share, and charge based on the age of each guest in your party.