And I'm not sure even those would be "highly polluted" these days (big Asian cities definitely are though - and I wouldn't want to raise my kids in New Delhi or Beijing).
My wife grew up in Vienna and my son will grow up in Berlin, I think both provide for a much more interesting childhood than the suburb I grew up in, and are not really polluted or dangerous (maybe some parts are, but we don't have to live downtown to get the benefits of the city).
Berlin in particular has a lot of late 1800s "railway suburbs" which have been politically integrated into the city-proper in the 20th century & are now just outer districts of the city.
These are plenty green and still well served by the city's rail and subway network, so you sort of get the best of both worlds (commute times are still longer compared to living in a more central district, but that highly depends on where in the city you work).
I will never complain about pollution again (I'm not saying any amount is good, but there are children all over the world living in much worse conditions than London at 17:30).
Large cities are no place to rise children they are cramped, expensive and highly polluted the kids may get exposed to a higher diversity of people but that is not necessary at a young age. As long as you raise your children with your own values and teach them to have and open and accepting mind it doesn't matter where they are.
I have no problem interacting with different kinds of people even though i only got exposed to such diversity after i graduated university and I could never imagine growing up in a large city it would have been awful.
Hell personally I'm trying to move to a smaller city close by to London and take up more remote work so i can actually have more breathing room.
I absolutely have the expensive, small and badly maintained spaces that tend to be the norm in large cities.
US cities are a bit different you could probably live comfortably if you are ok with needing to drive everywhere but that's ridiculous to. I don't have a driver's license I never needed one since everything I ever wanted was a walk away and if it was further I could just take public transportation even in the small city I grew up in.
Spending so much money maintaining and fuelling a car is a waste of resources.