We also had the "studio plus" on our last trip and I definitely would not call that a day bed. It was more like a wooden bench with a cushion on it (maybe 2 in. thick). I had heard a lot about it and was surprised when I saw it because I wouldn't have my child sleep on it...it just didn't seem comfortable at all. We stayed at the BC (not villas) many years ago and that room had more what I would call a day bed.
If I'm understanding you correctly, you have 3 kids with 1 under 3, but your under 3 one doesn't sleep in a crib. I understand your dilemna, because my DD didn't sleep in a crib after she was about 1 1/2. Quite honestly, the studio is going to be very crowded with that many people and your only other option would be to bring an airmattress although I'm not sure where it could be put if the sofabed is out. I agree that OKW might be the best option because there is more room in the studios where you could make a pallet or put an airmattress for your under 3 child.
Hope this helps!