I would think there could be a problem with informed consent ie, the dentist should have informed the parents that such restraints would be used if this was not common knowledge. That being said, it is common practice at least in the medical profession not to allow parents to be present during pediatric surgical procedures. I don’t think it would be helpful if parents were present during the removal of a brain tumor.
But the pediatric patient undergoing brain surgery is neither conscious nor restrained.