Asking which cruise line is best is always a question that bring out peoples passions.
Everyone has their favorite. Chose a set of ports you would like to see and then picka cruise line that gives you the best deal for those ports. Keep in mind that with a 4 to 5 day cruise your ports are going to be limited.
For the age ranges you speak of I would consider Royal Caribbean, Carnival, or NCL. All have good childern's programs. All have activities for the older ages too. You can look at the prices of each and make your choice.
If in your research you come up with questions, feel free to come back and answer them. If you can find a travel agent that is cruise centric, talk with them. But it never hurts to have done a little research on your one before you visit one so you know what to ask.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007