I can tell you now, the most important thing about building a tree house is don't do it alone. Even if you decide to build your platform at a height you can reach from the ground, I guarantee your going to need someone to help hold timbers in place whilst you level and fix it. Then when you get up on the platform, without a second pair of hands to pass you materials up, you will be up and down all day and it will take you twice as long to build. So my advice is always get help!
Trying to build without a design!
Right, you have your tree and you want to build your children the tree house of their dreams. Don't just jump in with lots of wood nails and ladders. People just try to make some sort of platform up in the tree and then end up sticking lots of old wooden planks on top of it... which looks like some sort of shanty-town appearing from your tree which is not a nice sight to look at for you or any neighbours that might be able to see it from their houses. So make sure you design your tree house before you start your project. With the right design, you can see what your tree house will look like and this will also help you make a materials list.
Spotting danger points!
You must know how to spot danger points when building a tree house. Remember, different ages of children require different safety aspects, I.E. the younger the children, the more safety required! Older children can climb ropes and ladders with confidence, but you wouldn't trust a toddler to do this so always think safety!
Movement of trees!
Most people forget when building a tree house, how much the tree actually moves... When you're building a tree house on a nice sunny day, it would seem fine to fix a main beam across from one branch to the other and screw it securely in place, thinking that it will not move. Well you would be wrong... if you fix it like this, when the wind picks up and starts blowing the tree around, the tree will bend and move. The tree is designed to take windy days but if you stick a solid structure on it or against it, things are going to brake when the wind gets up. So you need to know how your tree is moving, if you can watch your tree on a windy day to see how it moves, all the better. If you're going to fix main deck timbers to two different trees where they could move against each other or two branches that move in different directions when the wind blows, you need to fix only one end of the timber securely and have a flexible fixing on the other end to allow the trees to move. The are several different flexible fixings you can use!
Above all Safety!
Most people don't make tree houses safe, or even take care whilst building it. Most accidents happen whilst building your tree house, there's lots of precautions you can take i.e. when climbing up trees and fixing timbers, you might not have your platform to stand on. So just throw a rope over a high branch to what you are working on and tie yourself to one end and get that second pair of hands I was talking about to hold the other end, just in case you fall. Make sure you fix all the posts and hand rails securely and correctly.