How would you handle this tactfully?

My 8-year-old has been invited to a birthday party. Unfortunately, it is being conducted at one of these indoor parks that make patrons sign waivers indemnifying them against all claims including "negligence"! I've been trying to contact the park to see if they still have that policy, but I haven't been able to get through. I want to call up the parents of the birthday girl (whom I do not know from Adam) and try to raise awareness of this issue.

But what do I do after that? They might be aware of it; they might not be. They might have put down a deposit; they might not have. If they are already aware, it might not make a difference to them. If they don't scuttle their plans to use that park, I'm not sure what I should say at that point.

Also, please keep comments on topic. I'm not interested in the legal realities of waivers. I don't want to sue anyone. I just want the park to have a fear of God that I would sue them, and thus not be negligent.