It depends what the basis is for your fears. If you have just gone through a divorce and happened to read a few stories in the news about parental kidnapping, or watched a made for T.V. movie about a parent who kidnapped their child, stop and think through the situation. Has your ex ever done or said anything to make you worry that he or she would take off with the kids? How stable is your ex? Does he or she have a stable job or other ties to the area?
If your ex has threatened to take the children and flee, or if your children are saying things that lead you to think he or she may be thinking of taking the children, you should take action immediately. If you have evidence that your former spouse is considering taking the kids and fleeing, you need to let the court know.
You may wish to seek a court order granting you emergency temporary custody. You should consider taking the children’s passports and putting them somewhere that your ex cannot access. You will want to talk to authorities at your child’s school or day care to let them know that your children should not be picked up by anyone but you.