There are many many things to choose from as you already know, and most expensive. Most cruises don't go to Sitka, but if yours does the Fortress of the Bear is much better than I imagined for getting close to Brown Bears. About as close to seeing bears in the wild without being in the wild. Kids always like train rides. My cousin has captained for Dolphin tours, they know where the whales are and have fast jet boats to get you to the whales. The whales in Juneau especially in August are fantastic. Hopefully you will see bubble net feeding whales. Maybe the cruise will go up Tracy Arm Fiord to Sawyer glacier or Glacier bay. You'll see lots of glaciers when on the inland waterway from a distance. The Glaciers are breathtaking, but like watching grass grow when waiting for calving glaciers if they are not being active, probably pretty boring to young kids and they probably don't understand exactly what glaciers are and do.
Also in Juneau you might have time for Mendenhall Glacier. Look at the glacier, but spend time hiking on the elevated walkway adjacent to the visitors center, there are bears there that you can see from the walkway, sometime within feet of the bears. You will be 10 feet above them and safe. Most of the curio shops at each stop are the same, spent time looking in them at one stop but try to avoid at subsequent stops.
We were not on a cruise when we did these. We flew to Juneau and then used the state ferries to get to Skagway and to Sitka. You will have fun no matter, Alaska is a fantastic destination in the summer, that's the main thing.