Yes, people do think NZ would be one of the safest places in a nuclear war. We could be self-sufficient in food as well. But who really knows. Survivors of a nuclear war might well envy the dead.
Anyway, what your husband is suggesting is absolute madness. He wants you to immigrate to a country you have never even seen. I am a New Zealander. Lots of stuff on this thread by non-New Zealanders is plain wrong.
We are not in the 1950s. We legalised same sex marriage before the UK did. We have updated our euthanasia law when the UK still has not done so. We have had many female prime ministers. Our divorce law have been non-fault for decades. And all those years ago, we were the first country in the world to give women the vote - back in 1893.
Yes, New Zealand is a very sporty country in general. But not everybody is sporty, I'm not. We're not all out running the Coast to Coast or circumnavigating the country in a kayak though. My son, born and bred a New Zealander with his father's family coming to NZ in the 1870s, once shared a flat with two sports mad people. He said it was a relief to move out as all they ever talked about was kayaking. Most live in cities rather than rurally.
Many people come to NZ and they want it to be just like the UK but less crowded with better scenery. The idea that you could buy a very nice house with the proceeds of your UK house sale are probably long gone. Housing is expensive. Food is expensive. Medical care is not free although there is a public health system albeit with waiting lists and prioritising treatment. Going to the GP costs you money. Dental care is not free and is not cheap. Our public transport system is not great so being unable to drive would be a major inconvenience to you - most New Zealanders can drive.
Then there are the earthquakes. They are quite frightening. My boss once let me crawl under the table first being a gentleman while the building shook and we were on the fifth floor. Check out photographs of people belaying out of an office block in Christchurch after the Christchurch earthquake in 2011 and they were lucky. They survived and didn't lose limbs.
Also New Zealand does refuse visas for people with medical issues who might place a burden on the healthcare system. That is not unfair, we have to prioritise our own people. So your husband might be being a Pollyanna if there are medical issues with your son.
All that sun that people think is wonderful? Expect to coat your children in sunscreen if you are white on a very regular basis. I think we come second to Australia in skin cancer rates.
And as previous posters have said, if you do settle in NZ and divorce, you likely will not be able to take the children back to the UK. There are lots of women trapped by this - especially if they marry a New Zealander. This is not to say that the fathers that insist the children remain in New Zealand actually step up to do any childcare either.
I think people do come to New Zealand and settle here because they like it here. My own parents were immigrants in the 1950s. I think in your circumstances you would have to think long and hard about it.