Melatonin is an over-the-counter drug, so her status as an RN doesn't have much to do with the situation. Most states allow for those acting "in loco parentis" to make decisions in the parent's place.  That said, if there is no statute allowing the parent in question to act on your behalf--or if you explicitly told her she had no such authority then you could have a cause of action for battery (in a civil case, not a criminal case).That said, I don't really see what the big deal is here.  Melatonin is naturally produced in the body.  It is a natural supplement and was most likely given to help your 9 year old go to sleep.  If this situation made you uncomfortable (which clearly it did) then don't allow your child to go to that friend's house anymore. I don't think the use of the court system or the attorney fees you will incur are really valid here. Best of luck.