Look for caterpillar;they probably will know who does cable rigginng in your area.gobug said:Holding up a tree fort with your children inside is a pretty important job.
I would estimate the load and check the stats on the cable lable. The 1/4" vinyl is not necessarily stronger. It would be nice around the kids. My biggest worry would be the connections. If you use those little clamp thingys to connect the cable to the turnbuckle, they could work loose. You might take the cable to a shop with the tools to make permanent loops through the ends of the turnbuckle. The wire rope places will have the tool and it'll only cost a few bucks to have them make a safe connection. You will probably beat the home depot price on the cable and save a little towards proper connections.