Swings are great. I talk about playground and backyard swings in another post. But tree swings are their own special fun. It’s a little bit like flying.
I suggest you take your tree swing design seriously. For starters you need a solid tree branch on a solid tree that has no obstructions anywhere near the arc of your swing.
I used mountaineering gear for the rigging of ours. Seriously. You could climb Everest with the 10mm static rope and locking caribiners on our setup. Select a quality swing that can withstand the elements. If you don’t know knots and gear get knowledgeable advice. Your local specialty outdoor shop can be a source of expertise — and they’ll get a kick out of the fact that you want to do it right.
Test everything before you let kids get near it. When you’re ready to swing keep everybody out of the way. Swings need space!
Gravity is involved. Be careful. Have fun!