In one of the latest kidnapping scams in Shenzhen, a man surnamed Gu reportedly received a call from a strange number on Jan 6 and heard a boy crying. Kidnappers told him his child was being held and demanded a ransom of 120,000 yuan.

Gu immediately transferred 50,000 yuan to a bank account designated by the callers. But he realized he was cheated when he found his son safe in school after the money transfer.

After intensive investigations, Shenzhen police identified four suspects and arrested them in Fujian province one week later. The group was allegedly cheating parents of students in more than 20 similar cases in Shenzhen and Shanghai.

In other cases, suspects also told parents to remit money for the sake of their children. The threats were made through cell phone text messages, but no kidnappings were actually carried out in all of the 51 cases in Shenzhen, Deng Guangsheng said.

"The suspects acquired some information on the family members and threatened parents that they would hurt their children. But they did not hurt any children in the cases," Deng said.

"We are not rich but I will urge my daughter to come back home as soon as possible after school," full-time housewife Li Yachun said of her 9-year-old daughter.

Zhang Zhiwei, a lawyer and volunteer with nongovernmental organization Baby Come Home, said the government should do more to protect children.

"The number of child trafficking cases is also increasing, but only a few of them have been found and saved by police," he said.

Many parents looking for missing children go to groups such as Baby Come Home because of limited government resources, Zhang said.

"That might be a reason why some parents will still pay the so-called kidnappers, even if the danger comes from just one text message."