Beach Club is a great resort for that age. Stormalong Bay has an entire section of the pool complex dedicated to younger children. Kind of like a sand beach in and around a pool. The kids can play in the sand, wade into the pool, and have a lot of fun. Our 4 yo DS was able to walk throughout this entire section of the pool, so it doesn't get very deep. The location of the BC is wonderful, with easy access to Epcot and MGM. Soarin is our 4 year old's favorite ride now and he wanted to ride it again and again, so proximity to Epcot was a big benefit.
If you are looking for something in the MK area, the Wilderness Lodge has a large kiddie pool, along with great location and boat transportation to the MK, which our 4 year old absolutely loved. The Poly and GF have zero entry pools that can be waded into, along with great location and amenities.