Cool stuff I'm not paid to endorse: Playground compost plantings.

A company called Filtrexx makes those long rolls of burlap you see used for erosion control along freeways and constructions sites. They also make a similar product filled with compost that you can use for regular garden plantings, or you can lay them on a pile of dirt and make a grass-covered hill for kids to play on. They're making a play for the child care center market. Their on-line catalog is a slideshow.

It has to be replaced every few years, and it looks expensive to me, but I'm told by people who run centers that, as landscaping and planting goes, they're fairly cheap as well as being cool.