The criticisms arose after the Obama administration announced Friday it would not stop military aid to Yemen and South Sudan, where boys as young as 11 are enlisted, the Stars and Stripes reported Thursday.
The administration also added Libya to the list of countries granted waivers from the law and issued a partial waiver to the Democratic Republic of Congo.
Jo Becker, director of children's rights advocacy for Human Rights Watch, said progress in getting countries to stop using child soldiers has been uneven. While South Sudan agreed to stop using children in the military and began implementing some of the accepted steps, the practice hasn't stopped, she said.
In Yemen, there's been virtually no progress, she said.
Jesse Eaves, senior policy advisor for child protection at the Christian humanitarian organization World Vision, said Obama's decision was "really odd" in light of his remarks at the United Nations last week about human trafficking and the exploitation of children.
White House National Security spokesman Tommy Vietor claimed this week that progress had been made.
Burma, Somalia and South Sudan have agreed to eliminate child soldiers, actions that he noted "were a welcome achievement."
Granting the waiver to Libya, Vietor said, was done to support the country's "democratic transition."