I wouldn't bother getting the certified playground mulch. I would get what you like. I guess wood mulch wouldn't be good for bare feet so they will be wearing shoes. The large pine bark would not get in their shoes, that could bring into the house. If you have pets, dogs they might like to chew on the larger pine bark however..

If your concern is the shards and sliver-like pieces in standard cheap mulch, then have a look at any playground under the swings and see that your 3" underlayer will soon be mixed with the safer 3" top layer.

I would not layer them. As noted, the cheap mulch will quickly blend into and mix with the playground mulch and you will wind up with virtually the same effect as if you had just used the cheap mulch alone. Slivers and sharp shards are a concern for kids playing and not just on bare feet - kids fall all the time around playground structures.

If you can't go with the expense of the playground mulch, then look for wood chips, like arborist's wood chips, rather than bark. These tend to avoid the drawbacks of sharp and slivery edges.

Sand,pea gravel and shredded tires are also approved and cost less than properly processed wood chips. If you have a way of hauling bulk or can hire someone,contact a company named "Fibar". They are the largest distributor of playground mulch in the nation with outlets all over. If this is for private use where liability is not a concern, regular stall mats sold at farm and ranch supplies are as good as certified mats. If you decide to use non certified wood, do not use freshly chipped because at the needed depth it will heat up and quickly turn into compost or possibly undergo spontaneous combustion.

As a suggestion from personal experience, I chose to keep sod under the swings. When they're toddlers the swing is low and we are always there behind them, when a bit older they can hold the chains well and the swing height is set just above their dragging feet, and as tweens the swings are higher but they would jump off past any safety mulch anyway. That saved the expensive playground mulch to just around the playset tower and eliminated the mess of dragging feet and a huge unused area of mulch. I didn't have problems with killing the sod, but YMMV. If you have just a single-use swingset, it can be moved to avoid wear in one place.

" If you decide to use non certified wood, do not use freshly chipped because at the needed depth it will heat up and quickly turn into compost or possibly undergo spontaneous combustion."

LOL!! If that were true then a whole lot of off-leash dog parks here would go up as blazing infernos!! Untreated arborist's wood chips applied to a depth of 6-8 inches and renewed seasonally as needed are the standard surfacing material. Any "processing" of wood chips is not a requirement for use as a playground base material or for dog runs/parks or pathways!!

Even the megapiles of arborist's wood chips at landscape supply yards have little to no history of spontaneous combustion!!