What are the tree houses??
These are the treehouses.
They're purty, they're quiet, they're unique, they have 3 bedrooms (one of them being very small with bunk beds) and with a sleeper sofa and sleeper chair they sleep 9. But a: there aren't many of them, and they're a part of the timeshare program known as Disney Vacation Club, so they are hard to get. If you decide to rent one I wish you good luck but be prepared to hear "I'm sorry, theyr'e not available for the nights of your stay." b: they're expensive, prohibitively so for many people. And c: they're remote. People who stay in them who rely on Disney buses to get to parks must first take a shuttle bus to the main Saratoga Springs property in order to get a bus to a theme park.
The cabins at Fort Wilderness are very nice, too, and sleep 6, but they're main drawback is you only get 1 bathroom. I'm not sure how close the bathrooms for the other campers are, if you wanted to save time by having one or two people schlep over there to take a shower, but otherwise it might take quite a bit of time getting in and out of the cabin in the morning.
Some Disney Vacation Club resorts have 2 Bedroom Villas, which can sleep up to 8 and have 2 or 3 bathrooms in each. Some resorts, these Villas might be less than renting 2 individual studios, others the 2 studios might be less. Either way the difference could be negligible (within 20 bucks a day) or crazy different, depending on the time of year you go. But of course, since most Disney Vacation Club properties at WDW are attached to deluxe resorts (Animal Kingdom Villas, Bay Lake Tower Villas at Contemporary, Beach Club Villas, Wilderness Lodge Villas, Grand Floridian Villas and Boardwalk Villas), you may be able to find 2 single rooms in the regular NON-DVC section of those parks that would be cheaper than a 2BR Villa in the DVC section.
A Value suite at Art of Animation would accommodate 6, but with a master bedroom suite, and a second room with a pull out sofa, a pull out chair and a sort of Murphy bed. It could still be crowded for the other 4.
The "good news" is you wouldn't be allowed to be "squished" in a single room, as they're usually only allowed to accommodate 4 MAYBE 5 people plus a child under 3 in a crib.
In terms of affordability, your best bet would probably be 2 single rooms in a Value or Moderate resort. You could get adjoining rooms. The questions are whether or not you want to go "whole hog" on your trip and stay in one of the nicest resorts, and how important it would be for you to be all together in one unit versus 2 single rooms that may have a connecting door (would you want a full kitchen to make some meals, would you want your kids to sleep in a bed versus a folding couch, things like that).