There are few things that can drive fear into a parent's heart like an abducted child. Even if it is known or suspected that the kidnapper is the child's other parent, there is little comfort.
What can parents do in such dire situations?
The National Center for Missing & Exploited Children® (NCMEC) offers the following advice for those parents in distress.
As soon as you realize that a family member has abducted, or is planning to kidnap your child and remove him or her from your custody and the local jurisdiction, call 911 to report it. You should also call 1-800-THE-LOST (1-800-843-5678), which is the emergency number for NCMEC. The Center has resources that parents and family members can tap as they search for their beloved missing children.
Sometimes parental vigilance can alert them to the possible abduction of their children by an ex. Has the other parent attempted to gain access to birth certificates or passports? Has he or she made airline or other travel arrangements for domestic or international trips? This could be a red flag signaling potential parental kidnapping.
Here in California, the laws authorize investigators and prosecutors in the district attorney's office to initiate criminal and civil actions that may lead to the child being located either here in the United States or internationally. When the missing person is a minor child, there is no waiting period in order to make the report.
What if there is no custody order?
Regardless of the child's custody status, parents have the right to report their missing children. However, having a valid custody order in place can certainly be helpful. In some cases of international abductions, e.g., when using the Hague Convention on the Civil Aspects of International Child Abduction, a/k/a the "Hague Convention," parents may need proof of their custody of the child.
Whenever parents of children divorce, and also in the case of those who never marry, establishing custody should be a priority in order to most effectively parent their kids and advocate for their best interests.
Source: National Center for Missing & Exploited Children, "Family Abduction," accessed March 09, 2018