Love the idea of creating a natural "sand box". Let the children play: just add sand for really natural DIY sand box. I would just use a tarp to cover this play spaces when not in use to keep the cats out. Just add cinnamon to keep the ants out.
Natural Playgrounds Company built a slide into a constructed hill at an elementary school as shown here in Glens Falls, N. The embankment slide is safer than tower slides with ladders. Scattered boulders, random dirt steps, rough terrain and varied plan
A playground has ever been regarded as a location where children, by playing, learn how to turn into non-playing adults. Designing it should be fun! At length, playgrounds have to be responsive to every child.