Hi there. I'll try to give you a little help. We have been on many cruises so I do know a thing or two
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First of all, your cruise fare does include your meals and most entertainment (shows, etc). Extra charges would be for alcoholic drinks, pop (or soda as they call it). Usually when you eat in the dining room you can get water, iced tea or milk with your meal for no charge. If you or your kids drink a lot of pop, you can buy a soda card. In the buffet area they usually have the same selection of coffee, tea, iced tea, sometimes lemonade, (depends on the cruise line) and milk in cartons at breakfast.
Soda (pop) are free at the drink station on deck 9 (Magic & Wonder), I believe deck 12 for the Dream. Also free with meals. You do have to pay if you order from bar. 24 hr free room service (only remember to pay tip). All meals are free on the ship, except for Palo adult only restaurant (optional).
As far as when to book, do it as soon as you have decided which one you want to do. Some people will say wait for last minute deals, but March is a spring break month AND you are traveling with children. You want a cabin that will accommodate a family and those can be limited on some ships. You will usually put down a deposit at booking and your final payment won't be due until 75-60 days before the cruise. I have put deposits on cruises before and then changed my mind and got full refunds as long as it was outside the 75 day mark. (once again depends on the cruise line)
As this is your first cruise, I would recommend booking through a cruise specialist
travel agent. It doesn't cost you any more and they usually get the same price you would get online. You can go on travelocity and search your dates an 3 day cruises first to see what is out there.
Keep in mind that for a 3 day cruise you will likely be booking an older ship that while nice, won't have the "bells and whistles" that a newer larger ship would. The cruise lines tend to put their older, smaller ships on the short trips. So, while it is a good taste of what a cruise is like, it is a lot different on a 7 day cruise on a newer super ship.
Actually there is a brand new AMAZING Disney ship, the Dream 
that is making her maiden voyage in January 26th 2011 and will be making 3 $ 4 day cruise out of Port Canaveral.
http://disneycruise.disney.go.com/ships-activities/ships/dream/
Carnival is a great choice with kids as they have a great kids program and will be cheaper than Disney. Disney will be a nicer ship on a 3 day itinerary though. Carnival does 3 day cruises from Tampa.
Extra charges on all ships would be tips (usually about $10 per person per day; some bill less for kids), alcohol, pop, casinos, bingo, coffee from the specialty coffee bar, any shore excursions. That's all I can think of. We don't find we spend much at all on our cruises as we don't drink or gamble.
The kids programs are free, but it is nice to tip the camp counsellors at the end of the cruise.
there is no Casino on a Disney cruise. Kids program are free and the counselors are not in a tipping position. therefore they must turn their tip to the manager. The best way to thank them is to take the time to mention them by name to the manager. Many praises goes a long way for raises, promotion, etc.
Also, go over to cruisecritic.com. You will find the forums there really helpful.
HTH
Even though the ship may leave at 5 pm, you can board usually as early as 12 pm or 1 pm and certainly will need to be on by 3:30. Another option is to do the cruise after your Disney time. Might be nice to relax after running around Disney for a week.
