Most children's menus give a choice of an entree and then they can pick two items from a list of "sides" usually including things like fries, fruit like apples or grapes, packaged carrots, and some dessert type items like a cookie or ice cream. If they want dessert they would need to pick a dessert type item as one of the two allotted sides. But Disney is really hoping parents will pick apples and carrots...when the dining plan first started, the only available children's desserts were applesauce, yogurt and sugar free gelatin. They ended up adding back in some things like cookies.
Be Our Guest at lunch includes applesauce as a dessert in all its children's selections, so if your child wants a cupcake or cream puff that would need to be ordered separately.