My ex won't return the children. What do I do?
If you believe the children are in danger or are being taken away, call 911 now. If this is a refusal to follow a schedule rather than an emergency, do not force it in the moment. Document what happened and take it to the court that made the order.
Those two situations need opposite responses, so the first thing to establish is which one you are in.
If you cannot locate them, or you think they are being taken
Treat it as urgent. Call 911, and call the National Center for Missing and Exploited Children at 1-800-843-5678. They operate around the clock and work directly with law enforcement on family abductions. Say the words my child is missing. Federal law prohibits any police agency from making you wait before taking a missing-child report, and requires the child be entered into the national NCIC database within two hours, so do not accept being told to wait 24 hours.
Have your custody order ready or accessible on your phone. Say plainly if you believe the children are being taken out of the area or out of the country, and if abroad is a real possibility, contact the State Department's Office of Children's Issues the same day.
Then call your attorney the same day. Emergency motions exist for exactly this, and courts can act quickly, including ordering the children's return and restricting travel. Speed matters here more than in almost anything else in family law, and it matters most in the first day.
If it is a refusal, not an emergency
Far more common, and the temptation is to go and collect them yourself. Do not.
Do not force a handover. Turning up and creating a confrontation, especially in front of the children, produces a scene that will be described to a judge later, and the description will not favor whoever escalated.
Do not retaliate by withholding your own time or stopping support. Those are separate obligations, and breaking one because the other was broken leaves you in the wrong as well.
Do document it, immediately. Date, time, where you were, what was said, who saw it. A note written that evening is worth far more than a recollection three months later, and keeping a record covers how to write one that reads as evidence rather than as an argument.
Do ask once, in writing. A short message asking them to follow the order does two things: sometimes it works, and it shows you tried.
What the police will and will not do
This surprises people, so it is worth knowing before you call.
In an emergency, or where you believe a crime is occurring, police act. But many departments treat a disputed handover as a civil matter and will decline to remove a child from one parent to give to another, even with an order in hand. They may make a record of the incident, which is useful to you later, and they may attend to keep things calm.
That is not the officers being unhelpful. Enforcing a custody order is generally the court's job rather than theirs, which is why the route below matters.
The court's machinery here is faster than people expect. Every state except Massachusetts has adopted the UCCJEA, which includes an expedited enforcement process (a hearing can be set as soon as the next judicial day after service) and, where a child is in immediate danger of harm or of being removed from the state, a warrant directing law enforcement to take physical custody of the child. Ask your attorney about both by name.
The route that actually works
Take it back to the court that issued the order. Enforcing a decree or an order sets out what that looks like: courts can order compliance and hold a parent in contempt, and depending on your state can award make-up time or shift your attorney fees onto the parent who violated. Changing the arrangement itself is possible but goes through the ordinary modification standard rather than arriving automatically as a punishment.
Pattern matters more than any single incident. One missed handover rarely moves a judge. A documented series of them, with your own conduct clean throughout, is a different matter entirely and is what actually changes outcomes.
If your child is the one refusing
That is a different situation with a different answer, and treating it as defiance by your ex when it is not will cost you. A child refusing to go covers it.
What counts as an emergency, how quickly a court can hear one, and what police will do with a custody order vary by state and by department. If you cannot locate your children, do not wait for advice before calling 911.