Pick up a used tire from a local dealer - they will be happy to give it to you for free. Make sure there are no wires coming through the sides from the steel belts. If there are, pick another one. Scrub/wash it really well, as the tire black will come off on the kids' clothes if your don't. Paint if you like. Get a rope and tie it around/through the tire and again to the tree. Have fun!!
Hello. Are you attaching it to a tree? We recently purchased some nylon straps off Amazon for a baby swing. I believe they might be helpful for a tire swing/ropes too.
here is an old fashioned one.... https://www.youtube.com/watch?v=If8yJXCK4tc&t=42s