We've had a couple of holidays up there recently, one was a few days in Paihia, another time we stayed in Waipoua Forest and did a few walks in the forest plus some daytrips up to Hokianga etc, and the last was just camping at Kai Iwi Lakes. There are heaps of walks you can do, some nice cafe's and a few sights to see in the area.
In Paihia we did a daytrip to Russell and just walked around stopping for lunch somewhere (wheel the buggy straight onto the ferry), another excursion was out to Haruru Falls and Waitangi. The Treaty house is now free to enter if you are a NZ citizen and you can walk out to the Falls which is about an hour each way, or even walking from Paihia to Waitangi is nice along the waterfront. These are all walks you can do with a buggy. There are a lot of good food places in Russell and Paihia. We did a daytrip up to Keri Keri but there isn't a lot to see or do. We didn't go on any boat trips as Caleb was about 9 months old and we thought it would be a bit difficult.
The bus trip to Cape Reinga is usually better than driving because you go down 90 mile beach but you can't just stop when you need to so I would probably suggest driving over a couple of days with a baby that young.
There are about 3-4 walks you can do around Hokianga and Waipoua Forest that are an hour or so, and Rawene is quite a nice place to walk around. There is the odd little cafe around and the area that is in a great location to break your trip and many are pretty child friendly.
Sorry about the novel.