Experts chalk up the exodus of families to a number of concurrent trends. 

"There's no doubt that a cluster of extraordinarily successful companies and the massive wealth that they've created has impacted housing prices," says Karen Harris, managing director of macro trends at Bain & Company. This is especially apparent in tech and finance hubs like San Francisco, New York and Boston.

The kids haven't disappeared completely, but the families that do stay in cities are typically those that have the money to buy large homes and pay for private schools. "It's not that there aren't children in cities, it's that they're rich," says Harris."In fact, we've seen real renewals of cities with parks and museums and green spaces, but for the rich."