Wow, that's awesome that you guys have figured it all out!
I lived in Ohakune for 2 years... for the winter you'll all need hats, gloves, scarves, warm windproof jackets, warm socks, and waterproof boots/shoes in addition to the thermals. Then, when it snows, add waterproof trousers to that if you plan to go and play outside

There is very little inbetween the mountains and Waiouru, so when the wind is coming from that direction it is quite cold.
Since you are planning on moving in July (which is the time of year we initially moved to Ohakune) find out what sort of heating your new place will have... and if its got a wood-burning fire, you'll probably want to organise some firewood not long after getting there! The local papers have listings in the classifieds of people who sell firewood, and they often deliver. Alternatively, if you are on base, your neighbours should know who will give you a good deal.