The police are, I think understandably, hesitant to forcibly take children and give them to other people, despite court orders that seem to say they should.
To get the police to transfer a child from one person to another, the police need a very clear order stating what they should do.
The best order is a ‘pick-up order’ specifically directing the police to pick up a child and give the child to another person. That is, the order literally states "The police are hereby directed to pick up the child..." This usually results when a parent has refused to follow a prior order. I have never seen a case when a pick up order was not followed, although sometimes the police will work with the person who has the child to facilitate a transfer that is less hostile and stressful for the child.
The second best order is a custody order that specifically and unambiguously states that a person has custody during a certain day. If a police officer can determine that a person is not following a court order without good reason, the police officer may transfer custody. This is much more likely if the officer doesn’t have to use force or go into anyone’s home to get the child. But if the person withholding the child has a good reason to do so, such as an allegation that they have discovered that the other parent is abusing the child, the police officer is likely to leave things as they are and tell everyone to go to court and sort it out.
If the order is ambiguous in any way, or if there are multiple competing orders, or if people are giving reasonable reasons for not following the order, then the officer is not going to play judge and try to decipher it all. They are going to tell you it is a ‘civil matter,’ which is a shorthand for go to court and let a judge decide.