I didn’t have any string that was appropriate, so I took one of the LEGO wheel hubs that ride on the zipline to Menards and picked out some string that would work. I ended up buying #18 Nylon Masons Line.
I attached one end to his tree house bed with a screw eye. For the other end I needed to find a place to attach it that could handle the tension. I got lazy and just drilled a hole in his desk in a joint where the top meets the side and screwed the second screw eye in there. I figured I payed $12 for the desk and the hole is in a place where I can reality fill it later.
We quickly built a string rider and sent it down the line before bedtime. I can already see that I’ll either have to find a different place to attach the second end of the string, or build a wall so the string rider doesn’t hit the desk and fall off the end — kinda like the the mountaineer that falls off the mountain on the cliffhanger game on the Price Is Right.