I put a swing set in last summer and opted for just the grass. Both my kids have mold allergies, and so the regular mulch was out. I did have the rubber mulch around our sandbox, but it turned out to be really bad. It is dirty, even the expensive ones. And the rubber mulch just doesn't stay where you want it even with a border and then it ruins other grass. It also is not comfy after a few years it gets really hard and yucky. It gets dragged everywhere on the kids shoes and clothing, so when we put the swing-set in, I opted for nothing under it. After one year the grass still looks pretty good, but I expect in a few years it will be worn, but I don't really think it matters. I would rather deal with a bit of mud after the rain than the mess of the rubber mulch. As a side note, my parents get the most expensive mulch which is supposed to be best for mold allergies and they still get lots of mold and mushrooms it their beds, and need to change the mulch frequently. So I would opt for just grass, and deal with the mud issue when and if it happens. Oh and as far as safety goes, as long as there is no stones or gravel under it, I think grass or ground is just as soft as the mulch, maybe softer. And yes it is slippery after a storm, but all the grass is slippery and kids play on all the grass, that won't be affecting them on the swing set. My 3 year old fell off ours last fall it is a 5 foot one, and he was a bit winded, but fine. So I think you should be fine with just the grass under it. When he fell off the playset at the playground with mulch, he got splinters and cuts on his knees. It was pretty nasty.