I am not a fan of using tires, particularly around children. They will smear themselves against toys in ways adults don't. The tires are in the sun off-gassing. You can read about the diseases people get who work with tires.
Pallets can be made of treated wood, and that's treated with chemicals. I don't know how to tell a treated pallet froma non-treated pallet.
Don't little kids put marbles in their mouths? Seems like a choke hazard. Even if it's visiting little kids, it ought to be safe. Making stepping stones and embedding marbles for decoration would work.
Bark chips have a softening effect when kids fall off swings and poles.
We had a really big swing set when I was a kid, an A-frame style with 3" metal poles, the poles were deep in the ground in cement so it never shook or tipped. It was 8 feet across, the top pole was at 10 feet. The swings were made of thick hemp rope and plain 2x6s with notches for the rope. We could take the seat off and stand on just the rope. We would get going on the swings and jump off, see how far it would toss us, so the bark chips were way out beyond the swing set, maybe another 10 feet.
We could spin and slide on poles. There was an apple tree nearby for snacks. All the kids in our neighborhood had a blast on that swingset. We pinned blankets, sheets and 5x3 boards (we could carry them between 2 of us) against the poles to make forts. It was made out of very few materials, and our imaginations did the rest.
Don't play dumb with me! But you can try this tiny ad: