Followed some older cousins, fell off and busted his arm. Kids are guaranteed to find a way to hurt themselves. But tons of people never have any accidents on trampolines. (The only reason I would be nervous for a trampoline is if someone out of family got hurt and we'd be liable).
Anything kids do can cause injury. Sometimes I think we're so afraid of kids ever getting hurt that all the fun things for them to do will be taken away. Scooters also have caused tons of injuries, but my kids played with them for hours yesterday. Kids have to learn to play and yeah, sometimes they get hurt. Life just has risks.