"Children who live in poverty, especially young children, are more likely than their peers to have cognitive and behavioral difficulties, to complete fewer years of education, and, as they grow up, to experience more years of unemployment," the Census said.
In 2010, when the Census survey was conducted, 21.6 percent of children across the country were poor, compared to 20 percent in 2009.
That was mainly due to a rise in the number of children living below the federal poverty threshold, defined as an annual income of $22,314 for a family of four, to 15.7 million from 14.7 million in 2009.
The figures reflect the overall state of the economy. The national poverty rate stands at 15.3 percent and the unemployment rate is at 9 percent some two years after the recession that began in 2007 officially ended.
The number of people living in poverty has reached an all-time high in the United States, despite the country's position as one of the wealthiest in the world. Its gross domestic product per capita of $47,184 was 3,095 percent more than India's $1,477 in 2010.
In 24 states and Washington, D.C., more than 20 percent of those up to 17 years old lived at or below the poverty threshold.
The Census found that the percentage of white children in poverty increased in 25 states in 2010 from the year before.
Overall, "white and Asian children had poverty rates below the national average, while black children had the highest poverty rate at 38.2 percent," it said.
"The poverty rate for Hispanic children was 32.3 percent, and children identified with two or more races had 22.7 percent living in poverty."
Children in some states fared worse than in others.
"About one of every three children in poverty lived in one of the four most populous states, each of which saw increases in the number and the percentage of children in poverty between 2009 and 2010," the Census said.
There were 2 million children in poverty in California, followed by Texas, where 1.8 million children were considered poor. Slightly less than 1 million children lived in poverty in Florida and New York.
The Census found that the number and percentage of children in poverty rose in 27 states in 2010. New Mexico's rate increased the most, by 4.7 percent.
Among states, Mississippi had the highest proportion of children in poverty, 32.5 percent. In Washington, D.C., and in New Mexico, child poverty rates also neared one-third.
In 10 states child poverty rates are 25 percent or higher, including Alabama, Arkansas, Louisiana, South Carolina, Tennessee, Texas and West Virginia.
New Hampshire has the lowest child poverty rate at 10 percent.