First of all, go to the Holland America website and read their information on the childrens' programs offered. (click on "On Board Our Ships", scroll down and click on "Activities", then scroll down and click "Youth Activities".
I would also, as someone else mentioned, advise you to take a stroller. Even navigating around the ship can involve a lot of walking, plus the shore excursions, etc.
Every time I've been to Alaska, the weather has been gloriously cool (daytime in the 60's) and usually overcast and often as not, misty or drizzly. Not enough to really get wet, just damp! So be sure to take jackets and dress in layers.
Don't worry about the time change...the kids will probably adapt better than you! Just relax and have a good time! You will love, love, love Alaska! It is the most spectacular, breathtaking place I've ever seen! Enjoy!