They are teenagers.. they will survive. :-)
The issue I see with bringing the air mattress is space. With the sofa bed, the cabin stewards can open the sofa bed at night when they do the turn down service and close it back up to a sofa during the day. If you bring a mattress, you are going to be walking all over it everyday. Not entirely worth the trouble of dragging it along on a cruise versus making the teens sleep on a sofa bed.
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