I will tell u what we do. We put up cash so we can better monitor our spending. They are going to charge you 10 dollars a day per person..right off of the bat, for tips that are well worth the money...
as for buying the soda cards for the kids, skip the soda card and bring your own sodas on board. that is perfectly acceptable. but there is lemonade and fruit juice anytime the children wants it. you would not believe the people that line up at the pursers desks the night before disembarking complaining about their bill, and be assured it is their bill it just adds up very fast. I would never use a credit card.