Actually with most cruiselines if there is anyone in the cabin under the age of 21, then there has to be an adult 25 or older in the same cabin. Two 21 year olds can book a cabin, but a 21 year old and a 20 year old cannot. The only exception is if the couple is married and brings documentation to show.
Some of the cruiselines will allow the parents to book into one cabin with the children either next to them or across the hall. Some of the cruiselines also have the connecting cabins.
As far as booking into cabins then switching once you are on the ship, this can be done, but keep in mind that the shipboard credit is attached to whichever cabin you were originally assigned to. Also, most of the cruise lines on board cards with your ID is also the key to the cabin.