Keep in mind that the #1 expense for running a restaurant is the cost of labor. Remarkably, macaroni and cheese takes about the same effort to prepare and serve as roast turkey. The #2 expense for running a restaurant is the cost of customer acquisition, which at WDW is very often biased towards the children, so in effect children should be paying more in that regard. At many of these restaurants, the cost of ingredients is fourth on the list, behind the cost of facilities (rent, power, supplies). How much someone eats has very very little bearing on how much they should pay at a restaurant.
Folks need to remember that discounts for children 9 years old and under is just that -- a discount. Older children and teenagers pay "regular" price, simply because they're not eligible for the discount. Discounts are not required, nor are they expressions of generosity -- rather they are deliberate offerings intended to put forward a loss-leader in the expectation that that will increase overall revenues. When the amount of revenue gained by offering the discount fails to exceed the amount of revenue that would be generated without the discount, there is an obligation on business managers to not offer the discount. Evidently, the point beyond which the discount actually harms the company is somewhere around 10 years old.