The point about size is a good one. It's meant to be a cruise ship, one of the largest types of ship around and it's rather small. Why could it not be castle sized?
Hull could be in two pieces that clip together with a rubber seal to keep water out. First floor would have a state room with balcony at the back, two cabins with one having two rooms so that the children can stay in one and the parents in the other - the child's room would be half the size of the parents. One utility room like a kitchen, crew cabin, or ship's doctor.
Second floor would have restaurant, shopping arcade, entertainment stage at rear.
Third floor would have bar, swimming pool, bridge, captain's cabin.
Crew: Captain, first officer, "sailor", room service person, bar person, waiter.
Rooms, cabins would be modular so that their position in the ship could be changed.