Each person in the cabin gets a Sign & Sail card w/his or her name on it (its your cabin key), you basically put a credit card or cash on it for whatever amount you want. You would supply this info at check in prior to getting your cards, but if you choose to use cash you'd have to go to the pursers office to put down cash against the card. Each card has its own Folio# on it. This way you can tell who charged what on the account. I'm not sure if you can put different amounts on each card or not. I really wouldnt see why you couldnt as each card has its own number. Its my understanding the cards (w/all the Folio #'s) are charged to one cabin and on the same statement.
The card is your cabin key (and charge card) so if any of you loose it, go immediately to the pursers desk so that tehy can cancel that card and issue another one. This card is your charge card while on board. You can use it to make purchases in the gift shop, bars, lounges, shore excursions, photographs, etc. There's basically no place on the ship where cash is accepted (except for Bingo). Should you need to add more money to the cards you can at the pursers office.
Depending on how old the kids are you may not want to allow them to make purchases w/the card. I dont see where they would need it except to purchase soda's or mabie to purchase something in the gift shop. There are soda cards and they vary in price depending the length of the cruise (7 days was $19.) But dont invest in teh card unless your children drink alot of sodas. Soda's can be purchases @ $2.00 for a 20 oz bottle. So its up to you.
Have a wonderful time!!!