Edited to add: it has no bearing on what you will pay at the restaurant. You will pay for your food once you get there, based on what you order, not based on number of adults, etc. in your party. Whether you have the dining plan or not.
Unless, you book an ADR where you have to pay up front, in which case it will give you the option online to enter adults and children. Ie. Cinderella's castle or hoop-dee-do revue.