Hi Stephanie:
To me, the answer to the best Resort for someone traveling with a four month old is really going to be what Resort is most convenient for the parents or caregivers. I've been traveling to Walt Disney World with my boys, now 8 and 7, since they were infants. Ease of transportation was my main priority at that age. Since you are likely to spend time at Magic Kingdom, I would look at the three monorail Resorts, Contemporary, Grand Floridian, and the Polynesian. These Resorts provide extremely convenient access to Magic Kingdom and Epcot via monorail.
Other great Resorts known for their great locations and transportation options are the Resorts located in the Boardwalk area. Specifically, I would look at Beach or Yacht Club. Not only are these Resorts centrally located near the entertainment and restaurants of the Boardwalk area, but they are a quick walk away from Epcot, and a long walk or short boat ride away from Hollywood Studios. The Boardwalk Inn is also located in the same area.
For a value Resort, consider Art of Animation.
All of these Resorts will also be wonderful for your six year old as they have terrific pools (especially the Beach or Yacht Club) and restaurants.
Enjoy your vacation.