Our DSs took turns sleeping in the daybed and the other queen bed while we were at the Grand Floridian and they both said the daybed was comfortable. They will prepare the daybed for you when they come in to do the turndown service. Having 2 growing sons (both will be teens on our next trip) we love having that extra bed in the room. I believe the daybeds at the GF and the Polynesian are the same size as twin beds.
The AKL day bed is not a futon. It does in fact look like one if you are looking at a photo. It is not. It actually turns over the part that you sit your fanny on flips over and under to reveal a standard twin mattress. My SIL slept on it last August and said it was very comfortable. It is not a child sized, just regualr twin.