While my wife and I were walking around MK late one night this past week we ran into a manager and asked him how they decided who got to stay and the castle and about what time did they give it out. He told us that it is completely by random. The Dream team sit in a room and wait for the computer to spit out the results. It could be the 3rd person to come out of the rest room near space mountain or the 5th family with 1 boy and 1 girl, that way it's completely fare and by random. This is also how the Dream Fast Passes are given out.