Hi Nicole,
How exciting to be planning your first trip to Walt Disney World! It sounds like you are already an expert, knowing to take advantage of
Extra Magic Hours and to plan for your transportation to them!
I stayed at the treehouse villas at
Disney's Saratoga Springs Resort & Spa, and they are a lovely experience. They are secluded and peaceful, and I am sure that you will enjoy your time there. There are not buses that go directly from the treehouse villas to the theme parks. You will need to take a bus from the treehouse area to the main area of Disney's Saratoga Springs Resort & Spa, and catch a bus to your final destination from there. You could also walk from the treehouse villas to the main area of Disney's Saratoga Springs Resort & Spa; however, this is a fairly lengthy walk.
The good news is that
bus transportation to the Disney theme parks does start at 45 minutes before park opening. It will also be running in time to get guests to Extra Magic Hours. I suggest that when you check in at your resort, check with a cast member there about the buses running from the treehouses. They should be running early enough to get you from the treehouses to Disney's Saratoga Springs Resort & Spa and then to Magic Kingdom or Epcot; however, do check with the front desk for a time they will start running.
If you have a car, you might consider driving. You will be able to park your car very close to your treehouse, and then you can drive directly to the theme parks. As a guest at a Walt Disney World resort hotel, standard parking at the theme parks is complimentary.
Please do let us know if you have any further questions, Nicole!