http://www.nytimes.com/2011/07/19/science/19tierney.html?src=dayp

“Children need to encounter risks and overcome fears on the
playground,” said Ellen Sandseter, a professor of psychology at Queen Maud
University in Norway. “I think monkey bars and tall slides are great. As
playgrounds become more and more boring, these are some of the few features that
still can give children thrilling experiences with heights and high speed.”