Which is one of the deeply ironic things about urbanism as a movement. We have made our cities hostile and overly expensive to families. So people of color--who disproportionately are the folks still having kids--are decamping to the suburbs in large numbers: https://www.brookings.edu/blog/the-avenue/2015/07/31/black-f... https://www.brookings.edu/wp-content/uploads/2016/06/0504_ce... (p. 10). From 1990-2010, the percentage of african americans living in the suburbs increased from under 40% to over 50%. For Hispanics it increased from around 45% to 60%. For whites, by contrast, the percentage living in the suburbs was relatively flat.
My Bangladeshi immigrant family members moved to Queens in the 1990s, and as soon as they got their feet under them economically they moved to Long Island. That’s the story of big cities like NYC: https://www.nytimes.com/2019/04/18/upshot/how-much-slower-wo.... Large urban counties would be shrinking if it wasn’t for a continuous stream of international migration. (But of course eventually the immigrants become part of the stream of domestic migrants leaving the city.)
A large part of that is schooling, followed by crime wave paranoia. Most who have kids want to decamp to the best school district they can afford, even at the expense of other aspects of quality of life. Safety is an issue too, because a lot of Americans are irrationally paranoid about children being able to navigate by themselves. In Japan, a 7-year-old going to school by themselves by taking the train is normal; in the US, it's grounds for calling Child Protection Services.