This situation generally requires a court order. Assuming there is some basis for the fear that the children will be removed from the jurisdiction, generally the court will requirethat one of the attorneys hold the passports for the children in trust until further order of court. This minimizes the likelihood that a party will disappear with the children.
There is no way to completely ensure that flight will not occur. However, after a court order has been issued, the penalties for concealment of the children or removal of the children are very substantial.