choosing an NZ hike with kids

Depends a lot on the 12 year old. Ive seen some that could finish marathons and some that need a rest collecting the mail

Can shorten the last day, but that's an easy part of the trip. If going anti-clockwise, then you're going up from lakeside to Luxmore Hut with a full pack. Second day is continuous uphill slopes until you reach the steps toward the end of the 'Tops' section. Then it's a fairly rapid decent from the 'Tops' to the valley floor & Iris Burn Hut. It's a number of constructed stairways & steps & then a zig-zag winding track. Next day or 2 is just walking down the valley to Moturau & undulating lakeside depending which way you choose. First 2 days are a relative slog. I would say Routeburn or Milford are easier. Possibly better to start there ?. Even if you go Routeburn Falls Hut - Harris Saddle/Conical Hill, turn back to Routeburn Flats Hut - Back out to the car park you started from, as a 3-day 1/2 Routeburn trip ?. Paparoa is easy. Old Ghost Road is really good standard & bookable for certainty. Check out yr.no"insert place name". Very good long range weather forecast. Watch the trend for a few days as it's an educated guess, not a guarantee of some time in the future.