It will depend on your spending habits. We generally buy as much from as we can up front that we do not need to charge many things on board including things bar credits, soda package, photo tickets, and excursions. We also pre-pay the daily gratuity charge, because next to excursions and beverages (both soft & alcoholic) this can be the biggest expense during a cruise. At $11.50 per person per day, and yes it applies to children as well (at least those 2 yrs old & up), so it adds up very quickly.
Remember sodas & juices will cost about $2.26 per can/glass and a shake or smoothie just shy of $6.00. There are free drinks (iced tea, a juice blend, hot tea, hot chocolate, coffee, and ice water) available 24 hours a day from the Lido buffet, but who wants to constantly travel there or wind-up lugging travel cup all over the ship. The same free Lido self serve drinks can be ordered from room service and are available in the MDR during sit down meals. I do believe that iced tea and water are available freely in the kids camp rooms. There are water fountains located through out the ships. One way to control cost on soft drinks is to buy either the soda tickets from Gifts & Services prior to sailing or order the Bottomless Bubbles sticker (which covers juices and virgin soda blends like Shirley Temples and Roy Rogers as well) either from the Bon Voyage dept or on board.
Having the photos on board taken are free but if you wish to purchase them is where they get you at $19.99 per each 8x10 (which most of the better pics are printed in) and $9.99 for a 5x7 (the more casual pics). For the excursions just remember the pricing on those never drop just because a tour is not too popular on a particular cruise so if your interested in you may actual wind up saving by pre-purchasing in the My Activities section of Manage My Cruise since I have noticed they some times go up in price closer to the port day. Another option is to research reputable private tour companies for the ports you are visiting.
Charges to the Fun Shops and the arcade are general a family's biggest hidden expense if they don't place a daily charge limit on a child's S&S card. Bingo, the lottery cards, free cruise raffles, and the casino as well as the Spa and Salon can be downfall for the adults.
Remember your S&S account can be started with a credit/debit card then by the last night be paid in part or fully in cash.
That leaves your port spending but that need not always be in cash, most sit down restaurants & store front retailers will take a credit card which I like to use for items I want purchase protection on (more or less anything over $30). That means you would only need enough cash for smaller craft vendors, snacks & quick drinks, local transportation, tips for the tour guides, and possible for tours (some attractions are cash only) . As a word of caution only bring into port as much as you want to spend there & split it up between responsible adults to different carrying places like some in your pockets, some to a purse/ wallet, some in the camera &/or phone case. Just leave the rest secured in the cabin safe.
There's no need to go to your bank prior to cruising to get a large wad of smaller bills. The first couple of days guest services can readily change a $20 or even $100 in to smaller bills, as the cruise continues this gets a little harder to do but not impossible. Also the cage in the casino will gladly take a cash give anything including quarters for the launderettes.
Vacation Countdown Clock Generator
April 2013 Carnival Splendor
May 2012 Carnival Miracle July 2011 Carnival Glory
October 2010 Carnival Pride September 2010 Carnival Glory
Sept 2012 RCCI Explorer of the Seas Aug 2003 RCCI Sovereign of the Seas
July 1992 Premier Oceanic June 1992 Premier Oceanic May 1990 Premier Atlantic