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.