What do we know about children in poverty?
Official data for children under 18

In 2014, the poverty rate for children under 18 was 21%.  Children under 18 represented 33% of the poor.

Sources:
DeNavas-Walt, Carmen and Bernadette D. Proctor,  Income and Poverty in the United States: 2014 U.S. Census Bureau. Current Population Reports P60-252, U.S. Government Printing Office, Washington, DC, 2015. Accessed 10/19/2015