Want the real answer? He can keep them until a court date is set. For example even if there is a divorce decree with specific parental time the Police can not really do anything other than refer you to the family court and the (other party) can get in trouble for parental interface. However depending on the circumstances and the judges decision over why, what, or when the (other party) took the kids may depend on the final ruling or to a ruling that the you or (other party) file for child modification, ECT. Honestly I wouldn't involve the police unless the children are in danger. I would just go to court immediately because the Police usually can't do anything other than make a report and most do not even know how to read the court documents.
Mothers have complete and utter control over children all the rights unless a judge ruled otherwise in a court order. You should refer to a lawyer
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