We were not charged extra for GF waffles at numerous places (both QS and TS).
We paid OOP at 50s Prime Time, and DD was 9, but she really wanted the GF fried chicken. We figured we'd have to pay for an adult meal, but they charged a kids meal price, despite the fact that they (literally) gave her a whole chicken (yes, she got 2 beasts, two thighs, two wings, and 2 legs).
I guess it depends on where you go.
You'll never get charged extra when on the dining plan, though.
Also, if you have a child on the dining plan, and the child has an allergy, they can order something not on the kids menu in order to accomodate their allergy, for no additional charge.