This is for both of you:
When you rent, it is not temporarily yours. You are using someone else's property. You cannot insure the building, as you have no interest in it. You must have liability insurance, in case someone gets hurt inside. Even for an empty warehouse. The owner may also have to have some liability coverage. The owner will also need to insure the building against peril. What do you think his insurance company will say when he tells them it's being used by a bunch of kids with BMX bikes? The risk of there being some kind of loss, either fire, damage, vandalism, etc, is quite high. Now consider this: You own a building, and you let kids engage in a dangerous sport inside. The odds of someone getting hurt are very good. How do you think the insurance company will view that?
So it's back to the waiver question. You'd need an attorney who knows liability very well to write up your waiver. If the user is under 18, their adult or guardian would have to sign for them. Over 18, that user can sign. And even then, that won't stop anyone from suing. The building owner would need a copy of that waiver, and he'd still be stuck with legal fees. Got it?