What I found most interesting about this article is that parents felt so strongly about having their kids nap outside, despite the fact that the scientific evidence presented in the article that this is actually better than napping indoors is mixed at best.

The article cites a survey of parents who say their kids nap better outdoors, (with a broken link), but surveys are far less convincing than experiments. In this case, parents should not use this survey as a reason to have their kids nap outside, because the survey is just a reflection of their own opinions. Following it would be like eating their own dog food.

As for more scientific evidence, according to the article "Paediatrician Margareta Blennow says reports from the Swedish Environmental protection agency show conflicting results."

So from the article, it doesn't appear that there is substantial scientific evidence that sleeping outdoors is any better for the children. Why do parents continue to do so anyway? I think it's just a cultural thing. I think many cultures have basically random opinions along the lines of "Doing X is better for your health". As long as X isn't actually very bad for you, people will continue to do X despite the lack of evidence that it's actually good for you.