I find it confusing as well. I did find this info. The link is below.
ENHANCED PUNISHMENT FOR KIDNAPPING INVOLVING A MINOR
The Federal kidnapping crime provide special rules for offenses involving children. Where the victim is under the age of 18 and the offender has obtained the age of 18 and is not apparent grandparents brother sister aunt uncle or person having legal custody sentencing levels are increased to reflect the severity of the offense.
Under 18 U.S.C. § 1201(b) there is a presumption that if the defendant fails to release the victim within 24 hours after they are illegally seized, confined, or kidnapped there is a rebuttable presumption that the person has been transported in interstate or foreign commerce. This presumption was placed in the law to allow the FBI to be come involved in cases where a missing person has been missing for at least 24 hours.
FEDERAL KIDNAPPING CHARGES - OVERVIEW - Bradford Ladner LLP