Guests frequently ask about substituting an appetizer for dessert, and unfortunately, the official word
on this is no. The policy of WDW is not to substitute dessert for an
appetizer on the Disney Dining Plan.
I have heard that some guests have asked their servers to do this and
have been sometimes been successful in receiving an appetizer instead
of dessert, but this is not common occurrence and was due to the
kindness of an individual server willing to bend the rules a bit. In
general, you should assume that with the Dining Plan, dessert means
dessert.
Children on the Disney Dining Plan are required to order from the children's menu whenever one is available. In most cases, this is includes an entree, side dish, beverage, and dessert. Table service meals for children also include appetizers. You can read more about this topic
HERE and
HERE.
Enjoy your stay at Walt Disney World.