You'll need to creat a retaining wall/curb of some type on the low end to retain the granular material. Why gravel? it's heavy, expensive and more permanent (a pain to remove.) Wood based mulch is much more easy to get rid of when the time comes and provides the necessary soft landing.

Thanks so much. I had read that wood mulch would compact and degrade over time so it would not be ideal to put underneath the weight of a swing set. My plan was to put mulch on top, after putting the swing set up so I’d have the soft landing. I don’t plan on removing the swing set for the next 20 years at least. Would you still recommend mulch?

IME gravel gets kicked into the lawn where it can get flung by the lawn mower which can cause damage. It can also be flung by kids just being kids. So I also would suggest wood mulch or perhaps sand. Just add more as needed. Also, not rubber mulch which is sometimes suggested but has toxics and may nave metal splinters from belted radials.

I would stick with wood based mulch regardless of how long you keep the swings. It's your method of attaching play equipment to the ground that I'd reconsider. We're not in a position to say how to attach because we know no details of the equipment. Many swings are capable of rocking, bucking or even flipping when a youngster gets cranking on them. So having it just setting on the ground is probably not a good idea. I think some sort of metal/concrete anchoring would be better. And whatever mulch would be independent of the anchoring. It would be a mess at some point if covering gravel with wood based mulch. Too, sinking wood members below grade into gravel does not mean they're going to stay dry. They'll still be prone to rot like in soil, maybe just a bit slower.

Thanks. It is a Rainbow play system (the really heavy redwood ones) that is meant to sit on top of the ground without any anchoring. Even in their display rooms where tons of kids play on one set, they sit without anchoring. I had a couple of landscape companies out for quotes--they proposed leveling a 20'x20' square with gravel then putting some top soil down and then wood chips.