If your children have been taken out of the jurisdiction of the United Kingdom without the consent of all those with parental responsibility and or the leave of the court, this is considered to be child abduction under the Child Abduction Act 1984.
If you suspect your children will be removed from the jurisdiction, you can apply to the court for a prohibited steps order to prevent it, or for a child arrangements order, or for the child to be made a ward of court. It is also possible for a port alert to be activated and the police can also be informed.
If your children have already been taken, then the action you can take will depend on the country they have been taken to and whether or not the Hague Convention on Child Abduction or EU rules apply.
If you need further assistance with a potential or actual child abduction, please contact us.