If the child is located after child custody ends at age 18, than the taking parent can be put in jail, but the child cannot be returned to the left behind parent if the child does not want to reunited with the left behind parent. If the child is located before child custody ends at age 18 in the United States, than the child can be taken from the taking parent and given to the left behind parent. If the child is located in a on another country, than this would the depend on law of the country where the child is physical located (example China, India, Russia and Saudi Arabia).
Sorry, there are too many variables in this question to answer in this forum. Consult an international lawyer with child custody experience.
Years licensed, work experience, educationLegal community recognition
Peer endorsements, associations, awardsLegal thought leadership
Publications, speaking engagementsDiscipline