If this a confirmed or suspected case of parental abduction you need to consult an attorney. The invocation of u201crightsu201d is problematic unless the so-called u201crightsu201d are codified in law. So the real question is what, if any, laws have been broken. In theory The Hague Convention on the Civil Aspects of International Child Abduction is an international human rights treaty and legal mechanism to recover children abducted to another country but I wouldnu2019t count on it. You need to see an attorney, preferably one that specializes in these cases.General comments: If they are 18 or over with US passports there is no problem. If they are minor children a great deal depends on the custody arrangements (e.g. who has legal custody). The immigration status of the father may also make a difference, especially if returning with minor children. In all cases a notarized consent form from the non-accompanying parent is highly advisable. Free copies of consent forms can be found on the internet. Other documents (e.g. birth certificates) may be advisable. Because of hightened concerns about trafficking of minors and parental abduction both in the US and overseas it is best to check the US State Department International Travel website for information about current requirements for Mexico.