There is a home based zipline kit you can buy that has a note on top - it's the first thing you see when you open the box:
"Every zipline ends with a tree. Mind yours!"
That's good advice. I'd like to talk a little bit about the trees at the end of a zipline, or obstacle, or even your daughter's treehouse. The thing about a tree - any tree- that every builder has to remember is that a tree is not simply a large 10x10 plank or a telephone pole with bark on it. A tree is a living thing, and the safety of your structure is dependant on the tree remaining a living thing.
There are far too many "How-To" tree house books that attempt to explain the right way to built into a tree. And while the structures are viable, easy to construct and most likely safe, many of the designs I see completly fail to take into account the growth of the tree itself.
Remember elementray science - you can count how old a tree is by counting the rings inside it. Put another way, this means that trees add a new ring for every year of growth. So, that 4x4 beam you just nailed/lagged/bolted flush to trunk of the structure is going to retard the growth of the tree in the area of the board - where is the growth going to occur? A tree can compartamentalize and route around small damage, but if the damage is large enough you may very well end up with permanent damage to your main ground support.
I think this video shows what happens when tree based structures fail to take tree growth into account. I'm sure that when those eye bolts were added, the heads where on the outside of the limbs they were supposed to hold up (like in the picture on this post). See where they ended up in a few years time...