I would try to include the Queenstown area. There are lots of soft-adventure options suitable for active kids from there - day walks on the Routeburn Track, the Luge, horse trekking, low-risk white water rafting. Lots of accommodation & food options, direct flights from Oz, easy access to Milford / Doubtful Sounds, Wanaka area, Fox Glacier etc..
Are you keen on a bit of a road trip? If so, you could fly in & out of Queenstown, pick up a hire car & do a loop up to Christchurch via Mt Cook, across the alps to Greymouth via Arthurs Pass, down the West Coast to Fox Glacier (do the glacier walk & check out Gillespie's beach for glacier-fed debris), over Haast Pass to Wanaka & back to Queenstown. This would also allow a side trip to Milford Sound (great drive) and, depending on recovery from the earthquake, perhaps a trip up the East Coast from ChCh to Kaikoura for whale watching.
Depending on your interests, the North of the SI is also worthwhile (Marlborough Sounds, Able Tasman NP...).
Of course the volcanoes are in the Central NI near Lake Taupo & Rotorua. This is also a great area, but with 2 weeks, perhaps best saved for another trip, unless you want to skim and/or hop. Other parts of NI for consideration - Waitomo Caves, Bay of Islands, Coromandel Peninsula, West Coast surf beaches (Piha, Whatipu, Karekare), Auckland (beautiful harbour & strong South Pacific culture but sprawly), Wellington (more obviously cultural with Te Papa Museum & movie industry).
Can't go wrong really. Unless it rains.