Everything in this build was done using stuff I already owned in my garage. I'm certain there are better pieces to use, but this has been operational for 6 years without any problems.
Here is a clip of my 4 year old.
Step 1: Parts
2 good trees
50'- 3/8" cable
4 - Cable Clamps
2 - 1/2" threaded rod 4' long
2 - 1/2" tie
2 - 1/2" eye bolts
4 - 1/2" nuts
2 - 1/2" washers
2 - Electrical Box Covers
Handlebars from a bicycle
Pulley from garage door opener
Step 2: Put It Together
I've seen some people wrap the cable around the tree. Bad idea. You'll choke the tree and kill it.
I used a 3/4 spade bit on a 24" extension. I drilled half way through the tree when the moisture and pressure from the tree really got difficult to drill through. Next I dropped to a 5/8" spade bit too finish going through.
It took about an hour of drilling.
Connect the eye bolt to the threaded rod, push through the tree, add electrical plate, and then two nuts.
On the uphill side I sucked the eye bolt into the tree as much as I could.
On the downhill side I put the rod at max distance so that I could tension the system. I drilled a1/2" hole in a 2" PVC end piece and threaded the cable through it. this provided a stop that didn't damage the pulley or cable.
Connect the cable between the trees and tension with the down hill rod.
Step 3: Handles
On the cable is a pulley from a garage door with its bracket. I added double carabiners to lower the height until my kids grow.
I used handlebars. They work great!!! I drilled and bolted a U bolt to it.
Awesome addition is the rocket swing.
Step 4: Ladder
I used a pool ladder to get the smaller kids up there.
I sawed off the back half of the ladder. I removed the rubber stops from the bottom of the cut off part and put them over the cut ends.
I cut a 4x4 into two 6" pieces. I bored a hole in the top for the ladder to fit into. I used two 6" lag bolts to secure them to the tree.