I know I shouldn't interfere......

Villages can be friendlier with plenty to do, especially if there is a village school.
However, it depends on the age of the children and whether or not there is a senior school within easy reach.
The parents might have to be prepared to be taxi drivers.

A small rural town might be the ideal but it's up to them.
You just have to ? and ?

kircubbin2000 I think your experience was not particulalry common, although it could happen again.

If they now live and work in London, then they will presumably still be living in the near to London Home Counties, where so many houses are being built that families are constantly moving in and out and where high population densities, probably just mean they will be living in a large village or suburb. the children will go to schools where many other chldren are new comers. They will soon make friends and build up new networks.

About 20 houses were built in my village in the first 15 years we lived here. In the last 10 years about 300 have been added. Lots of new families moving from other places.

Country every time for time. Children are remarkably resilient and adaptable. They will be fine.

I was born in London in 1952 and apparently had lots of chest infections and spent time coughing. Eventually my parent were advised by their GP to move out of London. I was 3 at the time. I realise it’s not the same thing but for the children it would be far better for their long term health.

I know moving children can be difficult but the sooner it’s done, the sooner they can begin to settle. Personally I wouldn’t say anything and give whatever support you can whatever happens.

My children spent 20 years living in a remote cottage in Cornwall and loved it. I once asked DS about it because it didn't seem as if we had done anything very exciting during that time. His answer was "Have I ever suggested in any way that my childhood was not perfect" Trees to climb, endless bike rides, what's not to like? And they had friends too.

How old are the children? That makes a big difference. If they're younger, they'll be fine.

However, there has been a move recently back into cities. The rural idyll is often seen as too remote and cut off and very lacking the opportunities provided by a city. Plus huge commute times for parents, leaving children with limited contact with at least one parent.

I would do exactly as they are - there is no way I would bring my children up in a high pollution area. Your SIL is wisely taking the long view for the benefit of their health.

Just because you had a bad experience does not mean they will.