Hi Bonita,
I was lucky enough to stay in a treehouse villa at
Disney's Saratoga Springs Resort & Spa recently. My family of three shared the treehouse with a family of four, and we found the space to be ample and the overall experience to be relaxing.
We had three children who wanted to share the bedroom with bunk beds. This room is fairly small and the sleeper chair is substantial, so I do not think that you would be able to pull the sleeper chair into it. We brought a sleeping bag and the children took turns being in the bunk bed or on in the sleeping bag on the ground next to the bunk beds. This worked well for them!
As for bus system, there is not a bus that goes directly from the treehouses to other points at Walt Disney World. You will need to take a bus or walk from the treehouse section to the main area of Disney's Saratoga Springs Resort & Spa, where you will get off of the first bus and catch a bus to your destination. This can take a while, so I like your idea of renting a car. If you have a car with you, you will be able to park it very near to your treehouse villa. There is parking just a minute or so walk to each treehouse. As a guest staying at Walt Disney World resort, you will be able to park for free at any of the theme parks.
You can rent a car from the
Car Care Center at Walt Disney World. Check out the site I linked for information on shuttles to the Car Care Center. The unofficial website mousesavers.com also has information on other rental car options at or near Walt Disney World. Head to their website and search "rent a car," and look for the link entitled "How to rent a car at Walt Disney World."
Please let us know if you have any further questions, Bonita!