If the state board in your area allows it, I would request a written consent from her parent and that the parent be present, not in the room of course, but in the waiting room.
I agree, Parental consent is paramount. And by parental consent I mean a signature and a visual of the parent. I tend to think that 16 and up consent is not needed for facial waxing but better to be on the cautious side and apply this to all services for those under 18. In most cases waxing is a vanity service anyway so why take a chance!
I've not heard of any state disallowing Brazilian waxing services under a certain age. That would be a new one. I don't think it's so much a legal issue as it is an individual "choice" issue. Personally, for me they must be at least 18 to get a Brazilian, UNLESS the parent is requesting the service and is there to sign the consent form. Hope this helps.