It’s easy to create a city that works for families as long as you use economic redlining to exclude everyone who suffers from the social problems that affect every major U.S. city.
Differences breed contempt, or just displays the inability to empathize with a different way of life.
A sad fact of life. People with kids tend to regress back to their familiar mothership when the kids arrive. It's instinctual protection mechanism for their children.
Apart from that, cultural differences become much more salient when you’re a parent. Because culture is how we transmit our values—what we believe about how to live a good and successful life—to our children. Culture is also highly correlated with demographics and also is a source of conflict. My kids attend a private school with affluent white and black kids (about 2/3 and 1/3). The neighboring supermajority-black county is richer than our supermajority-white county, so there are no socioeconomic divides. But there’s quite a bit of racial conflict nonetheless. A couple of kids in my daughter’s friend group wanted to kick her out of a running group chat because they wanted to make it black-only (she looks fully south asian though she’s half white). My daughter is far more accepted by her white friends who aren’t socialized from an early age to have a sense of racial identity/group solidarity.
White/asian places like Brookline have cultural problems as well. My high school was about 30% asian, the remainder white (all affluent and educated). There was a lot of conflict over cheating and the school being a “pressure cooker” that boiled down to conflict between the asian immigrant proclivity of academically grinding to get ahead and the WASP mentality of raising “well rounded” students with diverse interests. Studies show that white families move out of school districts as asian families move in: https://www.the74million.org/article/fear-of-competition-res.... This is true even though, statistically, asians are more affluent than whites, much less likely to be arrested or imprisoned, etc.
Parenthood is where the rubber really hits the road in terms of any sociological preconceptions people may have. It’s the time when you’re faced with applying those ideas in practice in a situation that will have consequences for what you hold most dear.
This has happened over as little as a kid walking home alone or playing at the park while the mom is at an interview.
I've partially chosen to raise my family in a place with people like me because I know the neighbor kids drive trucks, work in construction, explore the backcountry on dirt bikes, etc from a very young age and I don't have to worry about the local nazi mom sending the child snatchers for the sin of a child having some independence.