You can break the flight up. Stop off somewhere like dubai and have a few days in a hotel with a pool, adjust to time zone. Then do the next leg. Or somewhere else logically en route.
Book night flights taking off 8/9pm. Bring pillows, pyjamas and things. Do ‘bedtime’ on the plane, brush teeth, story etc, each child gets a parent and sleeps on your lap. Trust me they’ll be out like a light.
We started long haul at 5 and 6 years old and honestly could have done younger. But we wouldn’t do 24 hours in one go, but we’ve done 12 hours a bunch of times.
Would small kids like New Zealand? I mean they won’t get as much out of it as older kids. But it’s an amazing place. So friendly. Food is amazing. If you like wine, wine is wonderful.
Off the top of my head some places they might like at this age -
Abel Tasman national park. Lovely long flat safe sandy beaches. Wonderful nature to explore.
Bay of Islands in the north - also beachy. You can go see dolphins.
Queenstown is cool - there is a lot of more outdoor pursuit stuff there they will be small for, but the wine country is lovely and there are wineries with play areas and things so mum and dad can have a nice lunch whilst kids play.
Can’t remember the name but there is a place with lots of little hills they filmed hobbiton for lord of the rings. There are beautiful glow worm caved which also kids would probably enjoy.
Wellington is a lovely kid friendly city where you could stop if you wanted a bit of civilisation, go to the movies or whatever.
Coromandel Peninsula beautiful, beachy, family friendly, lots of lovely still bays with caves to explore and rocks to jump in the water.
Maybe steer clear of the sounds (Marlborough and Milford). They are stunning but the experience is a lot about being on a boat or kayak and just experiencing the wonder of nature. Might be over their heads a bit and too young to kayak on deep water (unless you are super brave and one of you would take the little one out).
Steer clear of volcanoes. Again can’t remember the name but it’s where mount doom from lord of the rings filmed. It was a hard trek.
Eat out anywhere. Even in the sticks you will never have a bad meal. Buy good wetsuits. Even in boiling summer the water is freezing.
On that note - there is a lot of water in NZ. Both kids should take swimming lessons as most places are by sea or lakes. just so you can really enjoy / paddle in sea etc. I know going to places like this became way less stressful when my kids confident in water.