bullshit dude, i was a lil monkey myself when i was a kid and i still have a 26ft swing and rope from back in the day that's still going strong, hasn't stretched at all....
chains are expensive, heavy, and will fuck you up if they hit you in the face...
get some static climbing rope, it's sleek, super strong, wont stretch and will last for years... + it won't coil as much as chains will, so you wont even need a swivel as long as you occasionally spin to the other side....