As far as embarkation and disembarkation...
Embarkation has always gone smoothly for us in NYC. Just make sure to have the usual documents filled out, and it should go quickly. Note that there is no place to sit down in the terminal -- it's not all that welcoming, actually -- so be prepared for that.
Disembarkation, to put it mildly, can be nightmarish. You will have 2 other ships in port with you upon your return, and getting a taxi can take a long while. My trick, which I don't mind sharing with others, is to hail a taxi across the street. If you are driving, it is pretty easy as the parking garage is adjacent to the terminal, as JackieL noted.
I see you have young kids sailing with you. If they love the PBS show "Theodore the Tugboat," they are in for a treat in Halifax. (Halifax Harbour was the inspiration for the show.) There is an actual Theodore the Tugboat you can ride (either through the ship's shore excursion office or on your own), and the Maritime Museum of the Atlantic had a Theodore Tugboat exhibit consisting of many models of the "characters" in the show. The museum also has wonderful exhibits on Titanic and the Halifax Explosion.
We've been to Halifax several times, one of which was on Carnival, so I am happy to answer questions. (BTW, we are booked on Carnival Victory a few weeks after you.)