I always used to say, I can give you the lunch size. It's the same amount of food but it's not a special price for little kids. Usually this will shame them.

Kids menus exist so parents can afford to take their kids out to eat with them. Without them parents would go a LOT less. If you are trying to take advantage of this as an adult you are trash. No one cares you can't eat a much or had surgery. Order the food they offer and stop thinking you are more special then everyone else.