You chances of success are dependent upon the proof you are able to put before the Court. If it can be demonstrated and proved that they are not following a prescribed diet and routine, then you chances are very good. However, I caution you, you must have admissible proof. For this you will need the assistance of an attorney. You will most likely need expert testimony as well as proof the the grandparents are engaging in the conduct you allege. If will all depend upon the best interests of the children. I may be the Court will order the grandparents to follow the prescribed diet and routine or limit their visitation in such a way that eliminates the harm. There are a lot of things the Courts can do, including terminating the visitation. Again, it all depends on your admissible proof and what the Court determines is in the best interests of the children.
I agree with John, you need documented proof of harm. Have you taken them to a doctor right after their visit? What about a psychologist for the lying? Finally, what does the Permanent Parenting Plan say? You can always request to go back through mediation and modify the Permanent Parenting Plan to add those restrictions. Then, of they do not follow the restrictions, you can possibly Petition for Contempt.