Took a 5 and 11 year old last summer (taking them again this summer) here are a few notes:
They need their card to get on and off the ship at ports, they scan their card and then a parent card they are roomed with, so make sure you are there when they get on and off the ship as we had issues with kids in different rooms with different parents (they still let us one time but they had to look up the rooms were connected so it was more of a hassle)
They do not need their card for Camp Ocean but you need it to scan them in and out
You can ask for a ghost card for the room at guest services as long as both parties are there, but you can't "move" them to the other room