It varied with non-Disney restaurants that didn't have kid's appetizers on their menus. Some said they had no appetizers (Raglan Road and T Rex) and one let DS order an adult appetizer (Teppan Edo, which may be a Disney restaurant, not sure). Considering the kid appetizers are usually a simple salad, carrot and celery sticks, fruit or soup, I would go in thinking they will NOT be able to order any appetizer on the adult menu.