Try purchasing one of those key ring clips that you can clip to your belt. Once on board, the hole can be punched and put it through a key ring. That way it isnt around their neck and they wont lose it. The other way is to just hang on to it yourself. Think about it, if they are too young to watch themselves, where on the ship will they go that they need their sea pass card??
MARINER OF THE SEAS-JULY 2006
MONARCH OF THE SEAS-DECEMBER 2005
MARINER OF THE SEAS-JULY 2005
MONARCH OF THE SEAS-AUGUST 2004
They also sell lanyards on board. Most that Ive seen have a key ring that you can attach the card too. Ive also seen some people who have one the the card slides into a little pouch (like for id badges). How old are your kids? Ive always held on to my sons, but hes only 3. The main thing that they will need it for is getting on and off the ship, and opening the cabin door. You can arrange it so that they cant charge anything on it too.