Disney won't let families of greater than 3 book a king bed (with daybed) room.
Even if you WANTED to squish the two kids into the daybed, if they are both over 3 years old, you won't be allowed.
Daybed in the technical sense means a bed that, by day, is a couch. When you go to a furniture store, daybeds are twin sized beds with rails around three sides, creating a "sofa back". You usually use pillows to create a "couch". At night, you remove the pillows and sleep on the bed. My younger sister had one in her room when we were teenagers.
Disney's daybeds are smaller versions of the above. They are "almost" twin sized, but not quite. I would not be able to comfortably sleep in one and I'm only 5'2" and 130 lbs. I tried at the Contemporary, but there was no way I'd be okay with that. So, we doubled up in one of the queen beds. They are PERFECT for little kids, though.