Waivers signed by parents, releasing liability for future negligent acts committed against minor children in recreational and related settings, are generally enforced by judges in California as discussed here. 
Interestingly, a clear majority of courts in other states have held that a parent may not release a minor’s prospective claim for negligence.
And when the victim is an adult family member who dies participating in an activity for which he signed a waiver, those who can bring a lawsuit on his behalf will also likely be bound by the waiver.