It depends upon their interests, but I'll give you a few ideas. There is a wonderful Maritime Museum in Halifax that I recommend not only for the kids, but for the adults. There are exhibits on shipbuilding, Titanic, and Theodore the Tugboat (the show was based on Halifax Harbor), among others. Click
HERE for more on the exhibits. The museum is within a 10-15 minute walk from the dock.
Also in Halifax is Pier 21, the eastern Canadian immigration center from 1928-1971, This is right next to the cruise terminal. (In fact, part of it actually serves as the cruise terminal.) I don't know if this will be of interest to younger kids, yet one thing we enjoyed was purchasing a small booklet for about $1 (maybe a little more) that had room for stamps as we went through the museum. You'd go in the same order as a new arrival would.
If the kids are into forts, the Citadel is very interesting. Note that it is up a steep hill, followed by lots of stairs, so little legs might get tired. It might be best to grab a taxi to get there.
I'll be back with more later on Saint John. But Halifax can certainly be done on your own without a Carnival tour.