My DD and DS were 9 last year when we did Alaska on the Wonder. I loved it and chose doing Alaska with Disney because my friend took an Alaskan Cruise with RCCL or NCL and commented to me that she felt like it was a MUCH MUCH older crowd and she and her husband really didn't like it.
Our kids really enjoyed the cruise and I am glad that we did it while we were still able to hike (check out the Chilkroot hike and float - easy hike but beautiful) and while we still had the guts to do helicopter rides etc. . . We also did float planes to see the bears in Ketchikan -
We had done the Caribbean cruise the year before last, and the lure of Alaska was the only way to get DH to go along with another cruise last year

I am very happy we did it and the kids did get a kick out of the colder weather at the ports and the scenery and the whales etc. .
Having said all of that, I'll be honest, they were little and as much as you can try to impress on kids how truly amazing it is to be standing on a centuries old glacier, you can't expect an elementary school kid to fully grasp the magnitude of that - certainly not the same way a 40 year old would understand it. For example, the highlight of the trip for my DS was probably the Lumberjack show which was steps away from the port and cute, but cheesy. . . .

You know, they are kids! This is why I liked going with Disney so much. It gave my husband and I once in a lifetime experiences and the kids really had a good time. Whether they would enjoy it with another cruise line, who knows, but at least here you are sure that they will have lots of other kids their age to run around with as well as the Disney Characters. If they decide they don't care for the majesty of the scenery, you know they will definitely still enjoy the cruise.