I usually travel to Seward two times every summer. I take a day cruise almost every day that I am there. Both Kenai Fjords and Major Marine offer excellent cruises. I really enjoy the wildlife cruises that do not include a trip to one of the glaciers. You have calmer waters and great opportunities to see wildlife. Humpback whale sightings are fairly common on the wildlife cruises. However, humpbacks are not extremely acrobatic or active in the feeding grounds of Alaska. They are focused on eating, so if you see a humpback, you will probably see tail flukes when the whale is diving, exhalations when the whale is breathing, and the hump of its back as it goes under water. Having said that, I will report that last summer, I saw multiple breaches, pec slaps, and head slaps from a mother and calf while I was on a wildlife watch.
I have seen orcas on almost every cruise. I even saw transient orcas one time. Resident orcas that live in pods in the Seward area are much more common. Resident orcas do not eat other sea mammals. Sea otters, Dall porpoises, mountain goats, bald eagles, harbor seals, and Stellar sea lions are extremely easy to see. My niece and I were on one trip where we spotted an animal that our captain and crew had never seen on a cruise - we saw a porcupine. I have seen bears on many cruises. When they are on shore, the boat can get fairly close.
Major Marine has an optional meal on their cruises. Without the meal, their cruises are cheaper than Kenai Fjords. Kenai Fjords usually has a meal included. Both companies allow you to bring your own food. Skip the cruise that stops at Fox Island for a meal since you spend a lot of time getting off the boat, eating, and reboarding. Both companies have a National Park Ranger/Naturalist. Your children can participate in the Junior Ranger Program.
The longer cruises to the glaciers are also wonderful but may be a bit too much for younger children. If you really want the best experience, take the Captain's Choice cruise offered by Kenai Fjords. It is over $200 per person, but the boat is smaller, the number of passengers is lower, and you have more input into what you see and do.