The children of migrant workers are often targets
China is setting up a national DNA database to help trace missing children, as the authorities struggle to tackle people trafficking.
By the end of the month, a network of more than 200 DNA centres is due to be operational.
Thousands of children in China are stolen or sold each year.
Correspondents say the children of migrant workers are usually targeted. They are traded for a few hundred dollars and few are ever found.
At the moment, only 32 provincial DNA labs and 11 urban labs are connected. But the Ministry of Public Security says all 236 DNA labs in China will be connected by the end of May.
Samples come from 5 sources:
parents of confirmed missing children
parents who report children missing
children suspected of being kidnapped
children of unknown identity who beg on the streets
Parents who have lost their children have long been calling for the government to do more to help them, according to the BBC correspondent in Beijing, Quentin Sommerville.
Child trafficking is seen as a growing problem in China, despite government attempts to crack down on it.
In a society that favours male heirs, it is often boys who are taken.
The problem is exacerbated by strict birth control policies, which limit many couples to only one child.
Families sometimes buy trafficked women and children to use as extra labour and household servants.
There have been several high-profile cases of abducted children being rescued from mines and brick kilns - prompting a Chinese government campaign against slavery.
The general freedom of movement within China in recent years has made it easier for criminal gangs to operate - and for stolen children to be transported quickly and easily from one part of the country to another.
The new wealth in the country has also helped to make trafficking worthwhile, according to BBC Asia analyst Jill McGivering.
Initially, official efforts focused on cross-border trafficking, which China's politicians seemed more willing to acknowledge, our analyst says.
They directed some of the blame at foreigners and were cautious about admitting that Chinese gangs might be involved.
But as the scale of the problem and public anxiety have grown, the authorities have been forced to confront the issue more openly - and to reassure the public they are taking action, says our analyst.