Two bits of advice for you: teens are not considered adults, and you will need to book two cabins, one with each parent in it. (There must be at least one person age 25 or more in a cabin when there are minors cruising.) And Carnival has said recently that they do not intend to continue the last-minute booking "deals" - to encourage people to book earlier, they will be offering better deals for book-ahead people. (This is from the VP of sales.) A third bit of advice: work with a travel agent you like, who is familiar with cruises. They can keep an eye on the "deals" for you, once you let them know your preferences for travel dates, cruising style, etc., and can save you a great deal of time and effort on this score! Glad you had a good time!
P.S. Carnival isn't the only cruise line with good deals - see your T.A. for other suggestions.