Your on site options:
1. With 5 and an infant you actually can stay in one room where they have sleeping for five such as the Monorail MK resorts and Epcot resorts and some at PO--rule is 5 plus an infant in a crib is allowed in those rooms that will sleep 5. However, that would be a tight fit.
2. You can get two connecting rooms at any of the hotels.
3. You can try for a 2BR villa at OKW, BWV, VWL, or BCV which are the Disney Vacation Club resorts. Price through Disney is high. However, you can rent through a DVC member for significantly less than Disney charges--see DVC rent/trade board on this site. Those all have full kitchens, w/d, and two bathrooms and would be much larger than two connecting rooms at a hotel.
4. You can get a 1BR suite at some of the resorts (deluxes, Coronado; sometimes called "deluxe" rooms such as at WL) which have two queens in the bedroom and a pull-out full or queen (depending on where) in the living room. Those are usually expensive and two connecting rooms may be cheaper depending on resort.
5. You can get a cabin at FW which sleeps six--queen and bunks in bedroom and pull down double in living room. However, those are not large and 6 is actually a tight fit and there is only 1 bathroom. However, unlike 2BR villas and suites, the cabins often have discounted prices when discounts are generally available.