With 5, you will definitely need the two bedroom since your youngest is already 3. With DVC the studios are 4 plus 1 under 3, one bedroom is 4 plus 1 under 3, two bedroom is 8 plus 1 under 3 and Grand Villa is 12 plus 1 under 3.
While they do have the Studio Plus, you are not guaranteed it if you request a studio. We had one once when our son was 5. He hated that day bed. It's really more like a small bench with minimal cushioning. You would be extremely crowded in the studio with 5 people. We felt crowded with just the three of us.
You either need to do a two bedroom DVC or get a deluxe that holds 5. We stayed at the Polynesian with our son and two nieces; we had two queen beds and one twin sized day bed that was very comfortable.