Regarding the Fountain Fun Cards (pop cards, soda cards) whatever you wanna call them.... How much are they? Are they worth purchasing? And would my two teenage daughters be able to use one card between the two of them? And realistically I would most probably like to be able to use it as well for a coke for my rum and cokes to be used with the rum that somehow or another made it's way aboard in my luggage of all things.

My wife and I think they are worth it. If you drink 3 sodas a day they are. If not stick to buying it by the can.
At times they could share one, at the dinner table. At other places one would have to order, then give the card to the other one to go order.
If they catch them they would have to walk the plank.

The soda cards are issued per person, it would be hard for the girls to share one at the same time though. The price usually runs around $5.75 per day plus gratuities. So if you are going on a 7 night cruise the price would be around $40.00 for the week. You have to drink alot of sodas to make it worth that price. There are "free" drinks available such as lemon aid, fruit punch, water, milk, iced tea and coffee.

Another alternative is to bring a few soft drinks with you. Carnival will allow you to bring a reasonable amount onto the ship, but if you try to bring cases of them on board they will confiscate them.

We found the cards to be great when we were on our trip in April, one has to remember that dinner drinks will go in card also, so @ $ 2.00 pr can of soda, only need 3 drinks a day to be worth it,,, and you can use it by the pool when they come around for drinks,,,,