On the basic dining plan, no appetizer at TS is included for adults.
Children must order from the children's menu. Most, if not all, children's menu items are presented as three course meals due to WDW's current attempt to present "balanced" meals for children including all the food groups. The child's "appetizer" is generally something like chicken soup, raw vegetables or a small salad. They include it in the dining plan so it will be noted by guests that children may order three courses.
You should note also that the 2008 dining plan no longer includes a gratuity for table service meals, you may need to budget for leaving tips.