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You can also use a kids swing frame. If you do, concrete in the legs. Most of the home setups for kids swings, etc., use 4"X4" pressure treated. But what you're looking for is just to build a couple "letter A" setups and then put in a crosspiece to support the chains for the swing.

We find a few at the local flea markets, mostly built from oak. They run about $75 to $100 for a two seater (when you can find them). They really are easy to build from simple plans if you have a table saw to rip the slats. If you have a kiln nearby that dries hardwoods, they usually sell the samples they use to test the dryness for half price or less.

I built one back in 2006 from kiln dried Cherry wood. Cost was about $30 for the wood and 15 for the hardware. I have a few air hammers that do brads, finishing nails, and framing. Once I got the wood cut, it went together in one afternoon.

Good luck on your project. Give us some pictures when you get it done.