Speaking as one of the unspeakably selfish and “beyond cruel” adult children who emigrated to NZ - I would be surprised if either of my boys wanted to move back to the UK given the state it’s in, but I would support them wholeheartedly. I would try to find out why they felt they needed to move so far away and then do what I could to help.
When we moved, I was working 90+ h weeks with a long commute, often not seeing my babies awake for days in a row. If we’d stayed, we would both be working long hours and barely seeing our children. We certainly wouldn’t be seeing much of our parents or siblings.
Here, I work a much more respectable 55h week. My commute is down to a 10 minute cycle, I can often juggle my hours to do the school run, and we can afford for DH to work part time from home. So yes, by moving with the boys we have sacrificed their closer relationships with their grandparents, aunts, uncles and cousins - but in exchange for a much closer relationship with their parents. If it makes me selfish to prioritise my child’s relationship with me over that with my mother in law, then so be it.
There are other benefits - we do things here we’d never have done in the UK (multi day tramps sleeping in DOC huts), the schools where we live are all very good without paying exorbitant fees, the health care is much better than the NHS has become - but the biggest reason I stay here is because it enables me to be there for my children while they grow up in a way that would have been impossible in the UK.
My dad was upset of course but very supportive and has visited us here and seen why we love it.