Any kidnapping case is federal...but it doesn’t necessarily have to involve children.
Additionally, the feds only get involved when state lines have been crossed.
The Lindbergh Act is a federal law (48 Stat. 781) that makes it a crime to kidnap—for ransom, reward, or otherwise—and transport a victim from one state to another or to a foreign country, except in the case of a minor abducted by his or her parent.
The Lindbergh law provides that if the victim is not released within twenty-four hours after being kidnapped, there is a rebuttable presumption that he or she has been transported in interstate or foreign commerce.
The punishment for violation of the Lindbergh Act is imprisonment for a term of years or for life.