We cruised with Princess (Grand) 2 years ago. It was FABULOUS! They do an amazing job with their excursions, etc. The weather is COLD though! We went in early August, which was supposed to be the warmest time, but it often rained or was windy. No way I'd want to get into any sort of pool on that trip! I would also recommend that you pick a cruise that gets you into Glacier Bay. Many round trip cruises (from Seattle, for example), only take you into the smaller glaciers but not all the way to Glacier Bay. It was the highlight of our trip!
Also, if you go, I heard you should go South to North (end up in Anchorage), which is what we did. The scenery from Vancouver was green and warm, and then it got more and more icy as the trip went on. It was fabulous!
We have done lots of Caribbean cruises but Alaska was the most amazing of them all. VERY DIFFERENT from any warm-weather cruise, though! No matter when you go, it will be WAY colder. Also, less young kids.
Katherine