Hi Donna,
Welcome to the Moms Panel! I am guessing your travel party will be at least seven guests total, so you are looking at booking two staterooms. Thankfully Disney Cruise Line offers connecting staterooms, so although you book two cabins, you can leave the interior door open between the two for ultimate security and convenience.
Prices vary at different times of the year, but I will be upfront and say that the Christmas holiday week is one of the prime weeks to sail, and therefore most expensive. When booking cabins, it isn't really broken down between adults and children - the first two guests in each stateroom pay full fare, and each person thereafter is a reduced rate. So you will be paying four full fares (two full fares in each stateroom), and three "reduced rate" fares regardless of when you sail. I will mention though, if there is any chance you can sail prior to your little one's third birthday he/she will sail free (just have to pay the port taxes and fees).
If you decide to wait, you may want to explore this
special offer - beginning in January 2011 kids sail free on some of the 7-night caribbean cruises. You would still have to pay the four full fares (two per stateroom), but the three additional kids would sail free!
I hope this helps!