From pewhispanic.org/reports/report.php?ReportID=125:

An estimated 340,000 of the 4.3 million babies born in the United States in 2008 were the offspring of unauthorized immigrants, according to a new analysis of Census Bureau data by the Pew Hispanic Center.

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.