OP, our family (Me, DH and two teens) did our first Alaskan cruise last summer and I did a lot of planning. We chose NCL Pearl for several reasons and # 1 was the itinerary included Glacier Bay. # 2 was great pricing, # 3 was the option for anytime, casual dining and finally it was filled with other families.
On cruise critic website be sure to check out the NCL boards too to give yourself a fair comparison.
We did r/t out of Seattle for 7 days and while I would have loved to have done a one way itinerary the cost for flights back is huge. Flying into Seattle from Boston we had flight options of Delta, Jet Blue, etc. Doing a one way gave us very few flight options and the cost was huge!! Perhaps if we lived on the West Coast but for us it was a great first cruise and taste of Alaska. We know we will go back.
We stopped in all the same ports as Princess & Holland America and the Pearl is a very new ship with lots of activities for families and children.
Also, with a more casual dress code, we could pack a little less which is a factor for families, particularly with the size of any cabin. Alaska's summer weather really varies so you'll need to bring a fair amount already.....decide if more formal attire is really right for you.
You will need a passport so plan for that but it really doesnt take long to get them.
Every cruise line has it's fans but we really weren't locked into any particular line. I'm glad we chose NCL and our day at Glacier Bay was the highlight of our cruise.
Enjoy.