Well, both of your kids would be the adult price.
They have 3 levels of meal plan, the cheapest one only has counter service meals so no ADRs are needed. The others have table service so ADRs are recommended, but it depends on the time of year you go, the crowd levels, and if you really want the most popular places.
You need to look at the menus and crunch the numbers. We feel buying the plan is not worth it. My oldest is 18, a vegetarian (those are always the cheapest things on the menu) and hardly eats, wears a size 2. My youngest is 11 now and eats like a bird, no way would paying the adult fee be worth it for her. My dh is like a snake, he wants only one real meal a day, he feels sick if we eat too much.
We had free food once and it was great! We got to try a lot of the most popular places and we didn't care if we didn't eat all we got. We each got an appetizer and a dessert with our meals, NO WAY would we ever buy that much food. People who say "I saved $$$$ buying the meal plan" are looking at the prices of what they GOT, not what they would have bought without the plan.
You need to figure out if it is a good deal for your family or not.