According to the U.S Census Bureau, of the 308,745,538 people living in the United States in 2010, 12.3% were living below the poverty line. The graph below shows the percent of people living under the poverty line among single family homes and married couple families with kids under the ages of 5 and 18. Overall, there is a greater percentage of families with a single mother living in poverty than families with married couples.
Among families with a female householder, 45% with kids under 5 years old and 39.7% with kids under 18 years old are below the poverty line. Among married couple families, 6.3% with kids under 5 years old and 7.9% with kids under 18 years old are living below the poverty line. The percent difference between each household type is very large, but I am not surprised that more single parent households are below the poverty line because they only have one income to support the family. Additionally, I imagine that there are more single mothers with kids under the age of 5 living below the poverty line because they need to financially support their family while also providing childcare for their young kids. There are fewer married couple families living below the poverty threshold because it is possible that there are 2 incomes that support the family.