From the report:
Unauthorized immigrants comprise slightly more than 4% of the adult population of the U.S., but because they are relatively young and have high birthrates, their children make up a much larger share of both the newborn population (8%) and the child population (7% of those younger than age 18) in this country.
The figures are based on data from the U.S. Census Bureau's March 2009 Current Population Survey. The report comes at a time when much political attention is being focused on the U.S.-born children of undocumented immigrants, with some Republican leaders stating that they would like to revise the 14th Amendment of the United States Constitution so as to not grant these children automatic U.S. citizenship, a constitutional right for people born in this country.
The complete report can be viewed here.