China Labor Watch (CLW) said the supplier, Shinyang Electronics, was believed to have hired at least five workers under the age of 16.
The New York-based watchdog cited other violations, including unpaid overtime wages, excessive overtime and a lack of social insurance and training.
"We are urgently looking into the latest allegations and will take appropriate measures in accordance with our policies to prevent any cases of child labour in our suppliers," the South Korean Electronics giant said in a statement.
"No cases of child labour were found," it said.
China Labour Watch called Samsung's monitoring system "ineffective".
