Pavlovas are not easy to make at home. If you can make a good one, Piq, I'll invite myself to your place to try it.
We always introduced our homestay people to pav. One of our Koreans had two helpings and then licked the plate! Not just his. The one the pav. was made on!
I'd be inclined to make your holiday earlier rather than later. You don't mention snow in your itinerary, so presumably that doesn't feature in your plans, and the weather doesn't really get cold until July/August, although Global Warming does some funny things. Also, school holidays are at the end of August and run into September.
If you still want to do the Tongariro Crossing, you will need warm clothes and strong footwear. I'm not sure I'd want to try it with a pre-schooler in tow at any time, but you know the Altoid & I don't. Getting to Ruapehu's crater is a major exercise, and also not something for a pre-schooler, unless you want to pay for a helicopter ride. Altitude is about 2800 metres. The geysers are easy, though they involve a lot of walking and on a fine day you can get hot & tired. Also, take in a visit to Tamaki Bros. Maori Village at Rotorua. When we did it, dinner, tour & bus rides there & back cost $80 per adult, and we though it good value. (You two will do well in the obligatory sing-along on the bus on the way back Brush up some American, or even Californian, songs).
I hope you see Kea in the South Island. They are very inquisitive and have been ranked as one of the most intelligent animals on the planet, including dolphins & whales. I doubt if you'll see them eating rubber though. Those videos are made by leaving a car unattended for a while. Also, see if you can drive up the West Coast of the S.I. It's well worth it. Take the detour to Lake Kaniere too. Bring plenty of video tapes, discs or cards for your camera.
The ferry trip between islands takes about 3 hours. Cook Strait can cut up rough, so the trip can take longer. Most tourists who use a rental car drop it off at Wellington & pick up another in Picton, or vice versa as the case may be. The flight is about 20 minutes, plus time mucking about at the airport. The ferry ride is nice on a good day and if you have the time, I'd recommend it. Wellington harbour is quite attractive, and so are the Sounds on the other side of the Strait.
BTW, you do realise, don't you, that in flying from the U.S. to here, it takes 2 days to get here? On the other hand, the trip back has you arriving before you've left.
Let me know your dates, and I'll see if I can arrange a meeting.
We have no money; we must use our brains. -Ernest Rutherford.