Carnival is the only cruise line I know of that accomodates 5 people - I wouldn't be too worried about size of cabin - you're not going to be there that much and it's worth the squeeze to go on a cruise. But, that said, check into Carnival about their Quad inside rooms.
Also consider adjoining / connecting rooms. Usually an adult over 25 must be "listed" as being in a room booked with a minor, however, once everyone is checked in you can adjust the sleeping arrangements to suite the family. The suite or mini suite, while a great option, might require a bigger budget than you want to invest.
Let the 7 and 9 year old be "big kids" in their room while the 5 year old "sleeps" in your room.
You'll do great - and the kids will love the adventure. Now get out there and have fun!
I'm not knocking Carnival, and I'm not saying the suites above are particularly affordable--just that there are numerous options for those who have the money and need to comfortable fit a lot of people in a cabin.
We are booked for the Victory for 3/28 -- we have 3 kids ages, 13, 11 and 7 - we have opted for two outside Main deck adjoining rooms....two bathrooms, two tv's and a door when needed....I can't imagine one bathroom for the 5 of us (with the 11 and 13 year old being girls that spend tooooo much time in front of a mirror....) this way they will have their own and we have double the space....we are so excited !
My wife and I have traveled with our teenage son and daughter twice on the Fantasy without a problem. My son and I always went to the spa to get ready for the evening meal while my wife and daughter shared the cabin. The only other time we were in the cabin was sleeping. If your looking for major romance, get two cabins ( or leave the kids at home). If it's a family vacation, you can get along well in one cabin and spend your money on entertainment. Have fun!