S.G.
I am a firm believer in the rights of the parent to decide these matters. I believe that anyone including the state who would try to over rule a parent in this matter is over stepping thier bounds. Although I believe that, I also believe that as a business owner you have the right to set certain boundaries.
If this child was in school, it would be time for a parent teacher conference. You have the right to run your business the way you see fit. Someone has to make a decision. Either the parents will comply with your wishes, or you will be letting them go as clients.
As a business owner and a parent, I can see both sides of this issue. As a business owner though, I think it is important for you to take a stand and put it in writing, offer the parents a written time of compliance, and move forward from there. As a parent I would say under no circumstances should you feed this child foods they have not approved.
I hope this helps.