So it kind of depends what dining plan you're on.
If it's the Deluxe dining plan, then it doesn't matter, because all dining credits are pooled and they are not separated by adult/child.
If it's the regular Dining plan, then it does matter. The dining plan corresponds with your ticket - so disney adult ticket = disney adult dining plan. The reason most of the non-buffet sit down restaurants have to restrict ordering to the menu that matches their "category" has to do with the way they are reimbursed. Not every menu item is covered by the dining plan for the same reason.
This is exactly the reason why we did NOT get the dining plan for our trip when DS1 was 10. We paid OOP, and he was able to order the food he really wanted. And honestly we ended up spending a lot less than the dining plan would have cost us, so between that and my kiddo being content with his meals, it was worth it to pay OOP.