Hi, Kristen!
When ordering a kids meal at a quick service location, your child will get a choice of two sides, usually grapes, carrots, pudding, applesauce, french fries, or cookie. The second side counts as a "dessert" so they do not get to pick an additional dessert off the regular menu. If there is no kids menu available, they may order an adult quick service entree, drink, and dessert.
Additional desserts can be purchased and paid for separately. I think you will find that all the desserts get to be a LOT when you are on the Disney Dining Plan and that are plenty of sweet treats to go around without purchasing additional items.
Enjoy all the wonderful dining in Walt Disney World! For more details on the Disney Dining Plan, click
HERE.