That will be a great trip for the whole family and the kids will love it, plus you will get some fishing as well. You have a tow wagon and boat, so your team can travel around. The roads are not motorways, so take your time and ensure you enjoy the views. But the distances are not large.
The Ferry will unload you at Tryphena. There are quite a few lodges, apartments and rentals around Tryphena, and it is a good place to base yourself, plus there is a shop and some local watering holes. A strong westerly wind will not make Tryphena happy for boating, but you can drive to the East Coast to Medlands, which is a beaut beach especially for kids. The best weather tends to be November through to May.....these equinoxial winds make life tough at the moment. There is some great hiking and hotwater springs behind Whangaparapara - always plenty to do.
I have a mate who takes his caravan and boat and stays at the Medlands Camping ground, if the weather is not great for boating he becomes a beachcomber!. nice surf beach.
On the West Coast you should have no problem launching at Tryphena, Blind Bay and Whangaparapara Harbour. To visit Port Fitzroy you need to drive to Fitzroy (the only public ramp on that harbour).