Scarletjim- As you pointed out, you would not be the first people to have done the two cabins thing. You can certain ask them at the embarktion desk if you can have keys that open both cabins (I don't know if they can do this but I would think they could) If not that, then ask for three or four keys for each cabin so that you all can get into both and then sleep where you want to.
I don't think its a big deal to them if you want to switch cabin mates around as long as you are going on the cruise with them AND you booked the cabins, I do not know of any rules that say they have to have an adult in their cabin (especially if you are next door)
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