The Department of Homeland Security, U.S. Customs and Border Protection (CBP), in coordination with the U.S. Department of State and other federal agencies, has had for some time a program – which our office has frequently used - to prevent the departure of a child from the United States when presented with a valid, enforceable court order which prohibits the child's removal from the United States. 

Now the State Department has made official mention of the existence of the program.https://www.cbp.gov/travel/international-child-abduction-prevention-and-return-act

For more on this issue see my article International Travel with Children: Preventing International Child Abduction