"Back then, they just told me to keep looking," said Yuan Cheng, punctuating the sentence with a lengthy drag on his cigarette. Sitting in his mud-floored home in Hebei province, a few hours north of Beijing, the farmer is talking about the lack of interest from the police when his 15-year-old son, Xueyu, went missing from a construction site in Zhengzhou in 2007.

Six years on, Yuan says the police have finally admitted to him that there was a string of child abductions in the area around the time his son disappeared. But when he went to them, two days after Xueyu went missing, the police said: "Keep looking on your own and we'll talk about it again in a couple of days."

Tens of thousands of children are kidnapped in China each year for sale into adoption, street life, forced labour and prostitution.

The horror faced by parents whose children are stolen is highlighted in Chinese and international media whenever there is a particularly disturbing case. Recently police arrested a hospital doctor in Shaanxi province over her alleged role in stealing newborn babies and selling them. The police investigation managed to track down some of the missing babies and reunite them with their parents.

But that is an unusually happy ending in a country where parents say they are battling police indifference as well as traffickers in the hunt to find missing children.

In 2011, Chinese police rescued 8,660 abducted children, but it is likely that at least double that number were kidnapped. China does not release official figures relating to child trafficking, so estimates are based on the numbers of missing-child reports posted by parents online and of children reported rescued each year.

Estimates range from 10,000 kidnapped per year to as high as 70,000. Most parents who lose children stand very little chance of seeing them again.

At the national level, China takes child abduction very seriously. It has a national anti-kidnapping taskforce that investigates and infiltrates trafficking rings, and there are frequent anti-kidnapping campaigns that encourage citizens to report anything suspicious. But at local level, where the first, crucial reports will be made when a child goes missing, parents say the police just don't seem to care.

"The evening we reported it they went out and patrolled a bit, after that we never saw them looking [for her] again," said Zhu Cuifang, whose 12-year-old daughter, Lei Xiaoxia, went missing in 2011. The police also failed to check surveillance tapes at her school or interview any of her classmates.

Critics say that the slow reaction of local police plays into the hands of the traffickers. The involvement of organised rings means a kidnapped child could be taken thousands of miles and passed between numerous handlers over the first couple of days.

Pi Yijun, a professor at the Institute for Criminal Justice at the China University of Political Science and Law, says: "An important problem is that when a child is lost, the parents go and talk to the police, and the police need to judge whether the kid has got lost or has been kidnapped.

"At present, in Chinese law, they need to be missing for 24 hours to be listed as a missing person or as kidnapped, but that 24 hours is also the most crucial time – so there is a major conflict there. How can you judge quickly whether the child has got lost or is being hidden as a prank or really has been kidnapped? That's a serious problem."

Often, it is a problem that is never fully resolved. In rural areas and the outskirts of cities where migrant workers live, children aren't too difficult to acquire, adds Pi.

China's one child policy has created an environment where finding a buyer for a boy is rarely difficult; there are always parents somewhere who want a son to support them in their old age but don't want to pay the fines for additional children just to end up with more daughters.

Child kidnapping is so prevalent in China that even when a stolen child tells people what has happened, sometimes nothing is done.

Wang Qingshun was kidnapped and sold to "adoptive" parents in the 1980s. The couple who bought him already had two daughters and thought it would be easier to buy a son than keep trying to have one naturally.

While he was growing up, Wang told his neighbours that he had been kidnapped and that the people he lived with were not really his parents. But they didn't report this to the police until a decade later.

While individual stories of stolen children make the headlines briefly and then fade, parents never stop looking. Many say they are spending thousands of dollars searching, unsupported, for their children, fighting to raise awareness of cases that will never be solved.

In the six years that Yuan Cheng has been searching for his son, he has helped rescue other children who had been kidnapped and sold into forced labour, but he hasn't found Xueyu yet.

Zhu Cuifang and her husband, Lei Yong, haven't found Xiaoxia either. Still, they press on, because as Zhu put it, "if we can't find our daughter, life is meaningless".