Ah sorry I thought it detailed such issues.
Ok, Say the father has primary care of their child and the mother has her on weekends and not returned her then this is not abduction or kidnap (there are offences if Court orders are in place) but a civil matter. If the matter is civil then the police are in a fairly awkward situation and common Police policy would be to check on the welfare of the child and take appropriate action if required. If the parents are just being awkward then the police will try and negotiate a resolution but if that fails then the police are usually powerless.
There are many factors to consider in such cases to give a proper and accurate advice, however, this is a very common weekend issue.