I really hope you aren't blaming the park for your nephews busted elbow, or his buddies concussion.
........I also didn't realize you needed someone to TELL you to wear safety protection.. I did all those stupid things as well in my younger years (skateboarding, amateur BMX racing, motocross, etc) and did I get hurt? You bet your ass I did... Did you see my parents blame anyone for it? Of course not, it's called taking responsibility for yourself and your own actions.