A great book on this topic is How to Live Dangerously by Warwick Cairns. He tells the story of a school in Norway that conducted a similar playground experiment. In that study, they had two playgrounds -- a "safe one" with rubberized mats and chaperones keeping a watchful eye and a "dangerous" one with tall structures, concrete, piles of wood, metal poles and no adult supervision at all. Within a couple days, several kids in the dangerous playground had broken limbs or had minor scrapes, but they continued the experiment. Within a couple weeks, the injuries had subsided and the kids in the dangerous playground were exhibiting all sorts of positive behavior. Compared to the "safe" playground, these children were happier, healthier, and more socially developed and getting along better with each other.

I did all sorts of crazy stuff growing up. I'm thankful I had parents who gave me a large radius to roam and didn't freak out (too much) when they had to drive me to the emergency room on occasion. The radius for which kids are allowed to play outside has been shrinking dramatically over the past few years. Humans are notoriously bad at risk assessment. We're more worried about child abductions than childhood obesity. Statistically speaking, only one of those is a real issue today.