How about, "How dressed up are we going to get on formal night? I saw pictures on the cruise website and some people were in Tuxedos. I think that would be fun."
If they say they don't want to get that dressed up, you may be out of luck. You can rent a tux on board, but you need to reserve one in advance and you would have to go get measured for one so you know what size you need. So, you may have to come right out and tell them you'd like to try wearing the Tux on formal night because you are going to need their help in getting it arranged. If they say it would cost to much, you only choice there is to pay for the expense yourself but they can still help you get it arranged. I know if my child (and I have 13 year old) came to me and wanted to get that formal I would indulge them, so unless you have reason to think they are completely against it, there is no harm in saying so. No one you know but your parents are going to be on the cruise anyway, so what does it hurt for them to know you want to try the Tux?
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