I believe there is quite a bit of research on the benefits of green space for youth mental and physical health. Specific to your question though, One of Richard Louv's main points in The Last Child in the Woods: Saving our Children from Nature Deficit Disorder is the benefit of nature for children with ADHD. I have referenced one of the most recent articles I have read on the topic. I'm not sure this is what you had in mind, but a good read.
I've been working on a qualitative project with schools to investigate this issue. My co-author and I are currently looking into the links with the experience of 'dirt' and how that relates to the benefits of outdoor education.