My girls were the same way.. mac and cheese was all they would get until we started taking them on cruises. They started with the mac and cheese and maybe they would try something from the menu... by their second cruise, they were just ordering off the regular menu and even had foods that they "hoped" would be served on that cruise!
Try it with your picky eater. The waiters don't mind at all to bring them two meals (and it doesn't cost you anything to ween them from the kids menu this way)
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, Celebrity Silhouette 2013