Nice! I don't do school yards much. In this part of the world (So. Cal.) they are fenced off and the play areas have large rubber mats. Even the public parks are going that way under swings and rides.
I knew a fellow who had access to a contract worker who would detect after they dug out a lawn or razed a parking lot, where the top 6" of surface would be removed. He always did well! Unfortunately I didn't know him well enough to be invited on a hunt. They were "short notice" events any way.