Take the railroad excursion (Whitepass Railroad??) if you stop in Skagway. They can at least sleep in the train if they get tired. You will get a great scenic trip. Whale watching is also a good one. They can stay inside the boat and watch/rest while you get to see the whales.
Eventhing else might be a bit much for that age group (lots of hiking and such)
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