Hi Maureen and welcome to Cruise Chat!
For both destinations, I'd look at Royal Caribbean or Princess. Both do a good job for both adults and children, with great children's activities when you're aboard the ship, kid friendly menus, etc. You could tell the girls they'll be princesses on Princess!
As for whether they'll be bored in Alaska, you may want to go to those cruise lines' web sites and download a brochure of shore excursions offered in Alaska. You can judge by the activities whether the kids will enjoy it. I don't know your kids, but I think there are lots of unique and interesting activities awaiting the kids in Alaska, from panning for gold to dogsledding to a helicopter ride that lands on a glacier, to taking a train up a mountain, etc etc.
I would look at the Caribbean shore excursions, too. If you and the kids like water activities, nice beaches, and the like, there are tons of things to do in each of the islands you'll visit.
For Alaska, I prefer itineraries that are round trip from Vancouver, but look at the Seattle round trip itineraries, too. If getting to Seattle is significantly easier or cheaper, it certainly changes the preference.
You'll see whales and bald eagles from the ship, but the big ships don't get close enough to land to see much more than that. You will, however, probably spend the better part of a day floating near a major glacier, which is always a lot of fun. With a little preparation so you can explain glaciers to the girls, they'll get a lot out of that day.
Weather in Alaska is variable. In July, you will probably have a cold wind outdoors when the ship is sailing once you get to Alaska. Temperatures in the ports could well be in the 70s or even 80. It almost always rains at least part of the day in Juneau, and rain could also happen in Skagway and/or Ketchikan. They call it "liquid gold" up there. When you spend the day near the glacier, you'll want to spend time outdoors, with binoculars, and for that you'll need winter attire! Warm coats, hats, gloves. The most important thing about clothing in Alaska is to bring things you can layer, as it may start out cool in the morning but warm up quickly. You can always spend time in one of the observation lounges to watch the glacier (and the world!) go by. There will be a naturalist onboard, who will be broadcasting over the P.A. system during the glacier day, giving you all the history and geology of Alaska, glaciers, etc. It's fascinating!