Round trip from a US Port means Seattle although I think the Regal Princess (at least this year) is doing a round trip 10 day cruise from SF. Less scenic in the inside passage from Vancouver but still terrific. Round trip from Seattle, SF, or Vancouver means no Denali. The one way trips to/from Seward or Whittier are the cruises that you would use to add a land tour.
Good choice with Glacier bay. A must IMHO, especially for a first timer in Alaska. Spectacular! Photos don't do it justice. This will limit your choices of ships and lines too. Celebrity doesn't go to Glacier bay. HAL, NCL, and Princess do have itineraries to Glacier bay. Not sure about RCL.
Since rain will happen most likely sometime during the cruise I feel a covered balcony is a must. It seems the the newest Princess ships such as the Saphire and Diamond have many uncovered balconies. Balconies above you look right down onto your balcony as well. So if you're choosing Princess make sure of your location.
If you can't get to Denali and still want a train trip take the White Pass in Skagway. Great scenery and history combined. It's really nice.
There will be many kids on an Alaskan cruise. It is summertime after all. And in spite of any ship rules, the kids will be in the pools and jacuzzis.
Good luck with your research. An Alaskan cruise can't be beat.
