Get the kids involved with all aspects of the cruise. Have all of them read about the ship, the ports, embarkation and disembarkation, dining rooms and other food choices, kids/teen programs, and the travel arrangements to and from the ship. If you do this, you are preparing them for a very different kind of a vacation and they will better understand what is going on. A cruise is a great family vacation but you should get the kids prepared.
Hope that this helps.
And make it very clear that there will be no cranky behavior