A growing percentage of Massachusetts children are living in poverty, with more than a quarter of youngsters in Suffolk County and nearly a third in Hampden County coming from families with incomes below the federal poverty line in 2012, according to a new report that concludes that where residents live significantly affects their health and their longevity.

Even as the economy has rebounded, the data show that children in poor households have not enjoyed a similar recovery. In 2007, before the recession hit, 13 percent of the state’s children were living in poverty. By 2012 that had grown to 15 percent, and the rates remain higher now than in 2007 in every county except Suffolk, which includes Boston.

Childhood poverty is one of the key indicators of later health problems, concludes the report from the Robert Wood Johnson Foundation and the University of Wisconsin Population Health Institute. It’s their fifth annual examination of data for nearly all counties in the United States.

Physicians and researchers who care for and study poor children said the report jibes with what they have been seeing and worrying about.

“Families with young children are working harder and harder, but are falling farther behind,” said Dr. Megan Sandel, an associate professor of pediatrics and public health at Boston Medical Center.

Sandel tracks the number of young children treated in the hospital’s emergency room who are hungry and dangerously thin. She said recent cuts to the federal program that helps the poor afford food are forcing many of the families she treats to choose among rent, food, and heat, even as many reports suggest the economy is improving.

“We see these play out on the bodies of our patients,” Sandel said.

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