The softest material is tiny pieces of ground-up rubber, but most playgrounds don't use it because if it were to catch fire the smoke is toxic. I know from my mom being a teacher, the safest thing for a school playground is pea gravel-the little soft rocks. Wood chips will cause splinters if they fall on it & if they play on it barefoot. I'd also water it down before the kids go to play on it so it's not too hot to their little feet. As far as where to get it, I think Lowe's or Home Depot would have it.