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Here are two articles I found that explain the risks of having something like a skateboard ramp or trampoline in the backyard. There is definitely the potential for accidents to occur when teens and young adults use these kinds of equipment, and they should always wear protective equipment like helmets and pads to minimize the risks of serious injury.
As far as the waiver goes, I think you would have to ask a lawyer or someone from your insurance company to make sure that would stand up in court if someone got hurt.