WDW's rule is that if you paid the child's price, you purchase meals from the children's menu, where one exists.
In fact, they aren't supposed to let you buy three adult meals if you have 2 adults and one child on your reservation - the number of people you have will be listed on your key card. You're supposed to pay cash for the third adult meal.
Discussion of possible ways around this are not permitted on the DIS because it's against the clearly stated rules of Walt Disney World regarding the dining plan.