OP, it depends on your restrictions.
WDW is excellent at accomodating things like no sugar (diabetics), veggitarians, low sodium...stuff like that. You call the speical dining line and speak with someone about the restrictions and your reservations.
Now restrictions like gastric by pass or weight loss surgery they are a little less accomodating. Meaning at a buffet you will be charged full price...period. You cannot be charged for a "kids" portion or less than a regular adult.
Same thing with regular CS or TS places. You are charged normal, whether you eat it or not.
Hope that answers your questions.