Welcome to the Bunnings Workshop community @Holdent. It's terrific to have you join us, and many thanks for your question about building a treehouse.
I hate to say it, but I'd start by contacting your local council to find out whether a treehouse is something that is permitted. I have no doubt that in some locations, you might actually run into issues or at least some requirements you'll need to adhere to.
As you are working with an organic shape, your design really needs to be adjusted to suit. You might like to sketch out how you'll construct a solid floor base that suits the tree's branching. Typically this involves screwing structural timber together to place flooring onto. Once you worked out how to construct a floor within the limits of the branching, you'll then be able to frame any walls and add a roof to them.
I'd recommend you have a think about how structural timbers for a base can be placed and locked into position, then let us know any questions you have that we might be able to assist with. Check out @Dan_M's Building a treehouse discussion for some really good techniques.
Not wanting to start a sales pitch, but there are heaps of awesome cubby houses on offer that you might also like to consider.