We just went on this cruise in May. This is what I packed for my 2 boys (DH under packed and wound up buying more layers in port). We did this trip with 3 carryon suitcases and one checked bag (mostly for souvenirs I wanted to get, like the Ulu knife)
One pair of long underwear each
Wool socks
Underwear/PJs
One packable rainjacket
One packable down jacket (Uniqlo makes one for adults, but I got the kids theirs from North Face/H&M)
One fleece hoodie
A pair of water-repellant zip-off pants/shorts
One pair of jeans
One pair of sweatpants/joggers (for sea days)
One pair of waterproof hiking boots each
One pair of sneakers each
One pair of hiking sandals each (Keens)
One beanie hat
One pair of fleece convertible gloves/mittens each
One swimsuit
A button up shirt
T shirts
For myself I packed similarly except I included two dresses and a pair of nicer shoes, and leggings instead of the zip-off pants. I did laundry onboard twice because we spent two days in Vancouver before the cruise.
We wore all the layers on deck on glacier day, on the glacier helicopter tour in Skagway, and on the whale watching tour in Juneau (the coldest we were the whole trip was on the water that day!) Vancouver and Ketchikan were warm. With the exception of the small-boat glacier excursion that you can only book through Disney, all of our excursions were booked independently. We rented a Green Jeep and flew with Temsco to the glacier in Skagway, went whale watching with Captain Alan on the Scania in Juneau, and went flight seeing to the Misty Fjords in Skagway.
Hope that helps and hope you have an amazing trip!