My back yard is all sand what would be good to put down under a swing

Get floor mats that piece together like a puzzle. They have so many uses. Or try artificial grass. They sell it by the roll or can custom cut you one piece at a time.#home depot

I would use rubber mulch. It last forever, you don’t have to replace it every year. You can also get rubber boarders to go around the swing set to keep the mulch in.

One alternate would be rubber mulch, it comes in colors and very often used on school playgrounds. It lasts a lot longer than regular mulch and doesn't attract insects. Most major hardware stores sell it.

Rubber matting

https://www.lowes.com/pd/Rubberific-Brown-Paver-Common-16-in-x-16-in-Actual-16-in-x-16-in/3715056?cm_mmc=SCE_PLA-_-LawnGarden-_-PatioHardscape-_-3715056:Rubberific&CAWELAID=&kpid=3715056&CAGPSPN=pla&store_code=3159&k_clickID=9995ac91-6d4e-41d6-8dc5-1750f71debb2&gclid=EAIaIQobChMIs4rJgonC2QIVUVuGCh30Wwv5EAQYBCABEgJ3ZfD_BwE

I would not use recycled tire rubber mulch.... they are currently rethinking this as tires contain toxic chemicals including benzene which is carcinogenic

Mulch would be good,

A nice wood mulch. Larger pieces.