My kids are in a local club, and we took them rollerskating late last year. All the kids had to have the disclamer form filled in (by a parent) before they were allowed on the rink. I was there my self and filled in my kids form on the spot.
Thirty seconds later my kid (out of 42 kids) fell backwards off the skates and banged her head.
While she was dazed for a while and unable to partake in the rest of the activities she was ok.
It was only then I considered the rights and wrongs of signing the disclaimer.
I think if a child suffered a major trauma, at the event, both the club and the skate company would be jointly liable.