The measure was included in the Trafficking Victims Protection Reauthorization Act.
Human Rights Watch said the bill could affect six countries -- Afghanistan, Chad, the Democratic Republic of Congo, Sri Lanka, Sudan and Uganda. In 2008, those nations received a total of $3.5 million in assistance for military training.
"Government forces that continue to recruit children into their ranks are going to risk losing U.S. military assistance," said Jo Becker, children's rights advocate for Human Rights Watch. "U.S. weapons should not end up in the hands of children, nor should U.S. taxpayer dollars finance the exploitation of children in armed conflict."
A 2007 U.S. State Department report found that the government of Sri Lanka backs paramilitary groups that include children. The other six countries were found to recruit children into their armed forces.