Hi, I'm Heather, an attorney with 15 years family law experience.
Yes, in most cases, that's how it goes.
The police/law enforcement have quite a bit of discretion. Normally, when they have a dispute over custody, their main goal is to keep the peace. Alot of times, parents will then show the cops the court order that shows that one or the other is violating the order, and the police will just say, "it's a civil matter." Sometimes, officers will try to pressure a parent into turning over a child, because the court order says so, but they don't seem to normally force the parents to do anything, and by and large, they seem like they just want to keep the peace, and if the parents do not agree, they tell the parents to go home, and file something with the court.
This is why some court orders have a provision stating that law enforcement is directed to enforce the order. That way, the parents do not have to go back to court to get an order of enforcement. If the court order has a provision like that in it, the police will most assuredly enforce the order right then.
But you shouldn't violate the court order just because you don't think the police will enforce it right away. Police can usually find some reason to issue a ticket or to arrest, and it's possible that the police could arrest for "custodial interference" if you are violating a court order. It's just that from what I've seen, they usually don't unless there is an order for law enforcement to enforce the order, in addition to the order for custody.