There was a study done in Gothenburg, Sweden a few years ago that compared inner city kindergartens with those in the countryside. The difference was that in the countryside the children were found to be more confident. It seemed that the children in the city were discouraged from taking risks and hence never hurt themselves, no scraped knees, etc., while the children in the countryside were always playing in the mud, snow, trees, and so on, getting dirty, tired, and scraped.

The kindergarten (barnehage) my children attended in Norway was much like that, the playground had all sorts of mildly dangerous equipment that you could fall off of, tricycles that you could get run over by, puddles to jump in, mud to play with. All the children were pretty healthy and still are decades later.