This is exactly what I was wondering. Thank you so much for the information.The KTTW card will open just the room the person is booked in, but you can request additional cards from guest services to act as room keys. These additional cards won't work for charging or for boarding or exiting the ship in port.
Also, be aware that if your family is planning to split up when exiting the ship in port, you will need to fill out paperwork so that each parent can exit with the child which is not assigned to their room.