You can set a daily spending limit on each card through guest relations.
Alternatively you can setup their accounts as cash accounts by putting down a deposit. That option would allow you to set the top limit and if there was a reason you wanted to raise the limit you could give them the cash they could add to their account, providing they are old enough to handle money. Say one has reached the limit but you approve of them to play bingo in addition today.
Excursions can be paid with your card or prepaid with credit card through the cruise line beforehand.
11 cruises in 2012, 90 nights at sea visiting 15 countries
8 cruises in 2013, 56 nights at sea visiting 7 countries
Next cruise: None scheduled
PM me if you need a cruisemate for any Europe, transatlantic, or Panama Canal cruises
Date, ship, cost pp