When people ask about my rates, I always mention that it includes two snacks and lunch. If a child requires specific foods (ie. gluten free, lactose free milk, etc) it must be provided by the parents. I figure that's more than fair.

I've never had this situation, but I think I would probably let the mom know that if her child needs to eat above those times that she can provide extra snacks for him to eat throughout the day. Although, now that I'm writing this I am imagining all sorts of issues that it would cause at my place if one child is eating something and the others were not. I might be inclined to ask for a doctor's note that states that he needs to eat whenever he wants and the reason.

I also want to say that I flinched a little bit reading your post. It totally reads like she is ordering you around and dictating what you do and how you do it. Telling her son that you HAVE to feed him whenever he asks? Yikes. Lower rates aside, I think some boundaries need to be set up in this situation regarding what you will and will not do otherwise she's going to be calling you telling you you need to feed him sirloin steak three times a day at your expense. (that's an exaggeration, but you get my point.)