Backyard Playground: How to install loose flood? We are preparing a 17' x 17' area of our backyard...

We are preparing a 17' x 17' area of our backyard for a play nouns and plan to install 4 to 6 inches of loose fill (wood or rubber) for the surface. Should we verbs down into the ground to create a pit for the loose fill to be put within, or should be make the ground rank and use an above-the-ground barrier to hold within the fill?
Answers:    Best to do both. Dig down below the root stripe of the grass. If you don't you will have grass growing thru your crawl. Line the area near landscapers plastic, held in place near landscaping staples. Put your boundary marker from the bottom to 2 inches above ground level, next fill. Rubber is best as it won't present splinters and is more forgiving when landed on. Good Luck!!