GREAT question OP! We have been talking about doing Alaska in two years and I'm wondering the same thing.
We will have a nearly 11 year old daughter, 8.5 year old son, and 5 year old daughter and they loved the pools on our Wonder cruise and had the "best day ever" on Castaway. The hubs and I really want to do Alaska but worry they'll be disappointed not to have the tropical island experience. Don't me wrong, I think they'll be overjoyed to be taking a cruise and I'm sure they'll appreciate any trip, but I wonder how I can sell it to them all the neat things that we can do as a family.
I keep reading about all the amazing scenery and excursions and think they'll be plenty of exciting things to keep them entertained.
Looking forward to reading other responses.