You really should try to convince them to do Princess. They might be the youngest, but with an Alaskan cruise, the ship is not the big thing...the scenery is. Princess knows how to do Alaska. They have several lodges where you can do land/sea packages, including a trip on the Alaska Railroad, which literally drops you off across the street from the cruise port.
Denali is a must! If you are going all the way to Alaska, it would be a waste not to see it. Also, RC does not cruise to Glacier Bay National Park, which is also a must! (Princess does.)