This seems like a good thing. People might say, "Why only fourth graders?" I won't speak for them. It is good at any age to get off one's ass and into a national park (or state park or any park). I will speculate, though, that age 8-10 is developmentally a really good age to get kids out into parks -- before the distractions of adolescence and with more skills than the primary school years. Here's the last line of the article:

With the help of the National Parks Foundation, the Park Service will also provide grants for free transportation for schools that need it, and educational materials for teachers.