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 "[i]negligence[/i]"! 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 [i]want[/i] to sue anyone. I just want the park to have a fear of God that I [i]would[/i] sue them, and thus not be negligent.