If you're willing to stay offsite there are hundreds of houses that will suit your needs for well under $200 per night. Take a look at vrbo.com and homeaway.com
If you want to stay onsite, your options are going to be much more limited and much pricier.
It would be for a trip that my MIL wants to take (and will pay for). We have stayed off site before but she would like to look at options to have us all stay on site in the same suite.
I don't think a tree house would work, you would end up with at least one in the living room. There is a master with a king, one bedroom with a queen and a trundle, and a third with bunk beds.
We had a very comfortable trip in a three bedroom villa at Saratoga Springs Resort. MIL got the master suite downstairs. My family of three took one upstairs bedrooms, my BIL family of three took the other upstairs bedroom. We loved having the living room free, and loved the four full bathrooms, kitchen and dining room. We could have easily slept 10 without anyone in the living room.
Look into renting points via www.dvcrequest.com to save you tons of money on the reservation.
Not many options on site at all, unless you get separate rooms. You will be limited to Grand Villas, and the prices do match that title. Think $750 a night if not more.