The Chain by Adrian McKinty

You just dropped off your child at the bus stop. A panicked stranger calls your phone. Your child has been kidnapped, and the stranger explains that their child has also been kidnapped, by a completely different stranger. The only way to get your child back is to kidnap another child within 24 hours. Your child will be released only when the next victim’s parents kidnap yet another child, and most importantly, the stranger explains, if you don’t kidnap a child, or if the next parents don’t kidnap a child, your child will be murdered. You are now part of The Chain.