That's actually not true about paying children's prices at fixed price restaurants. They charge 100% by age with no exceptions. At non-fixed price restaurants you should be able to order what you'd like including just ordering a side or an appetizer or splitting meals (much better food than buying kids meals since kids meals are not the types of foods you want after GB surgery).
Since you posted this on the dining plan forum I'm assuming you want specifics for how to work the dining plan after GB surgery.
I would plan to split meals. At CS locations you most likely will be able to get a side instead of dessert making it even easier to split meals. At CS locations you can also use snack credits to order a side rather than a meal. At TS locations, the restaurants that are owned by Disney and are not fixed price will also allow you to split meals though they won't do the side instead of dessert I don't believe. Some non-Disney owned restaurants will allow you to split when using the dining plan but some won't so be sure to ask. You could always pay OOP for something small like a soup and then nibble off somebody else's plate at TS locations. At fixed price locations, you'll be using a credit regardless of how much you eat.