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Children are a disappearing presence in the Bay Area, the 2010 census shows, with slow growth or a net loss of the under-18 set in the region's nine counties, as families with young children move to areas with cheaper housing and better job opportunities. But diversity of the youth population is increasing as white and black families leave and are replaced by Asian and Latino families with young kids.

The drop in the overall population of children in the Bay Area would have been greater if not for the increased numbers of two particular groups: Asians and Latinos. In several counties, in fact, the loss of black and white children was mostly balanced out by the increase of Asian and Latino children.

Contra Costa County, for example, has 27,000 fewer white and 3,000 fewer black children, but 27,000 more Latinos and 7,800 more Asians. Napa County had a similar swap, losing 4,600 white kids but gaining 4,500 Latinos and 1,300 Asians for a net gain of 5 percent in their under-18 population.

In Alameda, the loss of 30,000 white and 17,300 black children was offset by an increase of 14,000 Asian and 16,000 Latino kids. Although the county lost 4 percent overall in its child-age population, it would have been significantly higher.

The Bay Area’s kid drain is a reflection of a statewide trend that was in evidence in the last census, says John Logan, a sociology professor at Brown University and director of the US2010 Project. “I thought it very revealing that statewide, California actually lost under-18 population of both blacks and non-Hispanic whites,” Logan said. “Young families are not moving to California.”