Hi,
We went to New Zealand this year with DD1 aged 3.5 and DD2 5 months. My brother and his family live in Auckland so we went to visit them mostly and had a great time. New Zealand IS very child friendly. Even "adult" environments such as wineries made our girls welcome.
We booked cottages on a weekly basis rather than travelling around as less disruptive for DD's and our main reason for going was to catch up with family. If you plan to travel then a van is a good idea but I've been told to check that you don't have to dismantle the beds every day as its a real PITA. Also there is a bit of a petty crime problem with vans being broken into. Certainly in the Waitakere Ranges we didn't leave anything in the car and the glove box open to show there was nothing to steal as there had been a spate of car break ins.
Summer on the North Island is HOT and certainly in the Auckland area humid. Even when we were there in March / April it was mid twenties and certainly warm enough for us.
A second for Rotorua really interesting area worth a visit.
We also visited Hawkes Bay. Great wine area with very nice vineyards to visit and Napier and Hastings are beautiful Art Deco cities.
Around Auckland the Zoo is just great a really nice size for our girls and lots of interactive stuff. We stumbled over a native wildlife encounter which was fantastic. 6 foot away from a live kiwi! Skytower is worth a look as is Kelly Tarltons. We also went Dolphin watching which whilst fun was pushing it a bit with the girls.
There are some accessiblish walks with young children in the Waitakeres. Some fab beaches like Karekare, where the Piano was filmed to explore but not swim in, and others like Cornwallis which are good for swimming. The Kiwis also have fantastic children's play areas usually with a great view attached.
As to what to pack, pack as little as possible. We took 72kg! with us and needless to say didn't use all of it. I had access to washing machines and driers though so could just wash what we used.
Anyway I've gone on a bit, sorry, but do go you'll enjoy it and its worth the journey hell to get there.