Children age 3-9 must order off the children's menu. All TS have a children's menu.
WDW child menus are presented as three courses. The first course is usually vegetables, chicken soup or salad in small portions - not something adults would generally consider an appetizer.
Because WDW recently reformulated the children's menus to provide a "balanced" meal including all the food groups, children on the basic dining plan get all three courses because that is how the children's meals are presented.
Guests over the age of 9 do not get an appetizer included in the basic dining plan.