Seeing as its a sand playground I would NOT go bringing crushed rock anywhere near it, the risk of mixing sand with crushed rock is somewhere over 99% and I would hate to have to replace the sand and dispose of the stuff that came out and the cost of that ?????
Sand is what you have to play with so support your trucks cranes and outriggers whatever on sand, you need lots of wood or steel turned into driveways and your trucks need to stay on them. Good thing is driveway wood or steel can be dragged from one end of truck to the other so if you are using a track loader for example a heavy steel plate can be moved around if truck movements are rare and the site is too large to cover every position scenario. The loader can smooth the sand before the pig plates are placed, that will help the pig plates to work without complication. Compaction of root zone is not an issue that you can avoid except by using pig plates so another vote for them. The sand has air in it, air won't hold a truck up, some compaction is inevitable, blow some air into the sand when you are finished.
Keeping the mulch out of the sand might be an issue, best to limit that to a small area or better do it where its benefial to the playground. Geofabric will help, staking it down is hardly necessary unless extremely windy, some rounds will hold it down well enough and not require cutting injury hazard treatment. Traces of mulch will rot to nothing in a year, crushed rock will remain forever, forget about using a complicated sandwich of potentially hostile materials.
Keep it Simple but go big, two layers of staggered overlapped steel pig plates and a track loader to position them. Hard to lose them in the sand and fully staggerred / overlapped they will support without bending or getting caught up under the trucks.
Vertigo
