I just go on www.disneycruise.disney.go.com/disneycruiseline and go through the motions of getting a quote. Today, my Dec. 28 cruise for a family of four does not come up as a choice, so I figure that it is full of families at this point. I am going back on to try for just two adults, since I read somewhere that they only book so many kids of certain ages, this way I will see if it is really full or just full of kids.
It is very difficult to get this. As far as I know this information isn't readily available. You can "pretend" to book a cruise through DCL, or other online travel agencies and see what comes up. It varies depending on the number of children you enter. I've done this twice on the same day, and got multiple answers even from DCL's site.
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