I teach my clients that a trampoline (pool, climbing wall, tree house, etc) is like a pit of snakes. If you dig a hole in your back yard and fill it full of venomous snakes, if someone got injured by those snakes, you're going to be liable. Waivers won't matter, signs in the yard won't matter, fences won't matter. You brought the snakes, you'll be liable.

The best protection if you choose to have a "pit of snakes" is high amounts of liability insurance. An Umbrella policy is generally very inexpensive and will absolutely provide protection if/when someone gets bitten.