If a parent refuses to return the children then it comes as quite a shock to people that, for the most part, the police will not get involved in parental disputes over children. In fact, the police do not have any jurisdiction in relation to Family Court matters. The police will normally advise you that you need to go the Family Court and obtain Orders. Even if you do have Child Custody Orders in place, the police will still not get involved in your dispute unless they are ordered by the Court (usually by a Recovery Order).
The police can, however, assist you by completing a safety check on the child (ie. they go to the house and assess the conditions, speak to the child if appropriate and ascertain if the child is in imminent danger). The police can also assist by attending with you to your previous residence in order to ‘keep the peace’. This would normally be utilised when you need to collect your belongings or furniture.