I knew Tim Gill from his book No fear: Growing up in a risk averse society. (highly recommend).
In his blog posting, why does nature matter to children? he offers a link to a very extensive literature review to discuss the empirical evidence of why nature matters to children. In general, he categorizes the benefits under 6 categories: Health, Well-being, Cognitive, Social, Emotional/behavioural, Ethical/attitudinal. If you ever need evidence to prove to your family, colleagues, school, community, and policy maker, this literature review is a great resource. Tim’s focus is on children’s play and free time and I love the way he challenges our views of childhood in a culture that is confused (in my idea) about the many definitions of ‘risk’ especially when it relates to children … Here’s his website http://rethinkingchildhood.com/ and he’s blogging too. Check it if that’s your quest too.