It was a picture that shocked viewers around the world: a Chinese toddler chained to a post outside a shopping centre in the freezing Beijing winter.

However, behind the image of two-year-old Jingdan lies a tale not of intentional cruelty but, it seems, one of misplaced love and fear: his sister disappeared from the same spot just two weeks ago.

"I was afraid I would lose him too," their father, Chen Chuanliu, said today.

Four-year-old Jinghong has not been seen since 22 January, when Chen left her playing with friends while he worked. Although Beijing is generally regarded as safe, he, like nearby residents, believes she has been abducted.

Tens of thousands of children go missing each year in China; most are the offspring of impoverished migrant workers like Chen, snatched and then sold on for anywhere between a few hundred and a few thousand pounds. Officials have warned that the problem is on the rise.

Boys are often sold to families desperate for an heir; girls can be reared as future brides for rural men. Both sexes are taken for labour or to beg for gangs, say experts.

The problem has grown so acute that authorities have launched a campaign to crack down on child trafficking.

Last year they created a national DNA database to reunite families and a website featuring the photographs and details of rescued children. Officials say they have saved around 2,000 children and returned at least some to their parents.

But Chen has little hope of reunion.

"It will be hard to find my daughter because I don't have a photo," he said– the family could not afford one.

"The police didn't tell me anything – I don't know whether they have opened a case or not. They just told me to look around the area myself."

Officers told local media this week that they were looking for Jinghong and were obtaining footage from surveillance cameras – well over a week after her father urged them to do so. The Guardian's calls rang unanswered.

"The police said they saw [the surveillance footage], but I'm not sure if they really did or not," he said. "I think the most valuable time for watching it was right after she went missing."

Chen is not an expressive man. When asked about Jinghong it was several minutes before he began to speak: "Her hair is messy, like mine, and quite short. She has a scar on her forehead. Her face is not round, but sort of egg-shaped, and she is about the height of this table.

"But kidnappers could change her clothes and clean her hair so she looks quite different."

Her birth was particularly precious because he and his wife were already in their late thirties. He supported the family by driving a motorcycle taxi, making 50 yuan (£4.60) a day.

His wife, Yang Caihong, has learning disabilities and is unable to care for the children, and the couple have no relatives in the city. Although the family have lived in Beijing for years, China's household registration system means they are still regarded as migrants and – unlike official residents – not entitled to free nursery care. So, from the age of just 40 days, Jinghong was taken to the shopping centre while her mother collected scrap there to sell to recyclers.

Chen tried to keep an eye on his children between fares, but people in the area worried. They bought milk and snacks to feed them up and returned Jinghong to her parents if she strayed.

"She was a very brave little girl – she dared to do all kinds of things. I think that was why she [could be] taken away," said Mr Zhang,a resident who believes she was abducted.

This week's publicity has led to a potential offer of free care at a private kindergarten for her little brother, Jingdan.

"This boy was noticed because of the locked chain," a local official told reporters. "But in fact there are many little children from migrant families roaming around in Beijing."