I agree with TrvlPro. CCL and RCCL are your best bets. If they are both going together, I would say to pick RCCL since they seem to be a bit more relaxed on age groups and they would be allowed to go to the same activities. I'm not sure CCL would do the same. Both of them have great programs with lot's of stuff to do.
RCCL may be a good choice too since if you are a HAL person, the RCCL ships are a little less party orientated and you may enjoy them more than CCL. My 13 year old perferred CCL but only because RCCL was letting older teens "invade" their activities. I'm not sure if most teens would care about that, but she did.
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