Chinese investigators have given two factories a week to rectify their "illegal practices", following reports that workers were treated like slave labour.
The factories are both contracted to produce merchandise for Microsoft.
A human rights group revealed last week how kids as young as 14 work 15-hour shifts and feel like "prisoners" making computer mice and webcams.
A National Labor Committee investigation found workers at the KYE Systems plant in southern China were forced to do excessive overtime despite being paid $0.65 per hour, reduced to $0.52 after charges for basic factory food. Shifts typically last from 7.45am to 10.55pm, the report said.
While investigators from the Dongguan human resources bureau denied any evidence of child labour, they found plenty of illegal activities
"We have not found evidence proving the two factories have employed any child labourer younger than 16, but the companies do have some illegal labour practices," a Xie Yanfang, spokesman told the China Daily.
Some 385 workers aged 16 to 18, and officially still at school, worked in the factories the investigation found. 326 were not been registered with local authorities.
Production line workers were forced to work an average of 280 hours in March, investigators found.
"The factories have been ordered to rectify these illegal practices within a week, or they will be fined or face other administrative penalties in accordance with the law," the spokesman said.
Not all agreed with the findings. "The NLC's report is pure slander," one factory was reported as saying. The report must have something to do with the Sino-American trade friction," he said.
Microsoft says it is still investigating the claims.