Here in the Lower Mainland of BC, it is against the safety regulations to have sand under a climbing structure or swing set. It is considered unsafe to land on. Falling from a good height onto sand can have the same impact as falling onto concrete as it is so compact. We are also required to have a 6 ft clearance at the end of a slide, and all around the swinging area. Good materials to use are bark mulch, pea gravel, and commercial playground surfacing tiles. The mulch and gravel also need to be so many inches deep depeding on the height of the structure. I believe it is up to 9 inches deep. But don't quote me, I don't have my papers handy