Regarding Permission To Bring Young Adults To Gym

My question concerns my right to bring my 15 yr old grandson to the gym with myself or any family relation..  I feel with a note from the parent giving permission for a family member to bring their child to gym should be enough to relieve PF of any liability of getting hurt onsite . If parent gives grandparent permission in writing that should be enough ..especially when childhood obesity is being addressed on Biggest Loser now ..If parent cant go and grandparent can why should an overweight child suffer ..