My friends and colleagues,
Also, please take into account the security of the tree. I'm sure there is an arborist website, somewhere, that willl provide information on how to not "choke" the tree branch. I have seen the damage to a tree from a wire wrapped around the trunk. A tree is a living thing that will continue to grow. If you tie a knot that will throttle the branch, it could kill the branch, and have you looking for a new place to hang your swing from. Worst case, it will cause the branch to fail in a windstorm or some other high stress event, like when Dad goes for a swing.
Pat