What an absolutely thrilling experience! My wife and I brought our 5 year old son out of the whale watch boat, but told him it was just a "boat ride" in case we didn't see any whales. While it was quite a long ride (hour out or so) before we got to the area where the whales are, our little guy was so thrilled with the speed of the boat and spotting birds, boats, islands and lobster buoys.
We saw a total of three humpback whales, and two large schools of Atlantic White-Sided dolphins. One of the humpback whales approached the boat so closely we could feel the spray of his blowhole! Apparently when they approach so close, they're curious about the boat, and it's a gesture appropriately called a "mugging."
At one point, both pods of dolphins, and all three whales were all together near our boat. It was hard to know where to look and what to watch!
The crew were wonderful and knowledgeable.
One piece of advice - bring lunch and/or cash. The galley has some options, but they run out quickly and are cash only. Lastly, wear and bring sunscreen. Even if it's cloudy/rainy on the mainland, it might not be once you're an hour out to sea!