I would also agree with Kuki, we did the whale watching in Juneau with Capt. Larry last Sept. Part of the 2-1/2 hours is first off a bus ride to the dock, its not the same one where the cruiseships are, then at least another 1/2 hour to get to the "right" spot to see the whales and a good hour at least out there. He may move around some to get you as close to them as possible, but at any point they could come right by the boat. YOu can go up-top, which is great and just be amazed when you spot them, or hear them.
I think the kids would love it, you'll be suprised at how fast the time goes by, not enough time to be bored and Capt. Larry is very imformative, just make sure when you book that you ask to be on
his boat, they have I think 3 different ones, depending on how many bookings.
Have Fun, its definitely worth it to do this in Juneau :-)
Pic taken while on Capt. Larry's whalte watching boat: