The claims were made Monday by China Labor Watch, which said seven children under the age of 16 — and perhaps many more — were working at HEG Electronics’ plant at Huizhou.
The plant assembles cell phones, DVD players, portable music players and stereo equipment for South Korea-based Samsung.
New York-based CLW, a group set up to pressure corporations to improve conditions for workers, says it carried out three investigations during June and July 2012 and found instances not only of child labour but also of exploitation of students.
"Samsung Electronics has conducted two separate on-site inspections on HEG’s working conditions this year," the company said in a news release, "but found no irregularities on those occasions."
"Given the report," it said, "we will conduct another field survey at the earliest possible time to ensure our previous inspections have been based on full information and to take appropriate measures to correct any problems that may surface."
CLW said it found the seven children "working in the same department (at HEG) as our investigators. This suggests that child labour is a common practice in the factory."
"The number of underage workers throughout the factory is unknown," it continued, "because our investigators had limited contact with workers in other departments. But the company has clearly violated Chinese labour laws."
"These children were working under same harsh conditions as adult workers, but were paid only 70 per cent of the wages when compared with the formal employees. Moreover, these child workers were often required to carry out dangerous tasks that resulted in injury."
CLW estimated that there may have been 50 to 100 children working at HEG, with the youngest being 14 years old.
It said its research indicated that student workers make up as much as 80 per cent of the factory workforce.
CLW accused HEG of having employees work 11-hour days, including as much as five hours of forced overtime, six days per week and from 26 to 28 days per month.
"There are extremely strict punishments, and the workers are frequently fined," it said, adding that "night shift workers are only given time to eat one meal during the 11-hour work shift. The normal meal break is 30-40 minutes long."
It also alleged working conditions are dangerous and injuries common.
HEG Electronics, according to its website, is also a supplier for Motorola and LG.
After Samsung announced it would inspect the plant, CLW warned Samsung that HEG would evacuate its child workers and said that inspectors may need to "carry out in-depth interviews and thorough investigations, such as examining the number of the working positions in the factory to see if there is a sudden shortage of workers."
It said it has not contacted Chinese government agencies because it only wants the children to be able to return to school.
Previously, CLW has also claimed that there are problems with Apple Inc.'s Chinese suppliers, including an explosion in December in which it said 61 workers were injured.
Its latest report said conditions at HEG are "well below" those at Apple’s suppliers.
And in 2010, CLW accused Foxconn Technology Group of running a sweatshop in Taiwan where conditions were so bad that they led to a number of suicides by workers.
Foxconn assembles iPhones and iPads for Apple. It has denied the charges.