When you go into the playground, go all the way to the right-hand side, and you should find it. If you go there, and look at your map, you'll see a "little dip" on the map. It's the only protruding section in that part of the map if I remember right.
Now, in addition to that.. if you looked for 15 minutes and you can't find it... do you have the glasses which let you see invisible chests? If not, then you will not be able to see it.