Two 10-year-old children have been working unpaid shifts in a McDonald's restaurant in Kentucky until 2am, an investigation has found.

The US Department of Labour (DoL) said the children, who have not been named for legal reasons, prepared and served meals at a Louisville outfit of the fast food chain.

The DoL added that Bauer Food, LLC, a Louisville-based operator of 10 McDonald's franchise locations across two states, employed the youngsters.

Both children were discovered to have worked the cash register and drive-thru, and sometimes cleaned the floors, with one child even allowed to operate the deep fat fryer.

They were also not paid for their shifts, which would sometimes go on until 2am, the investigation found.

The children also had access to "dangerous equipment", banned for employees under the age of 16.

Bauer Food franchise owner Sean Bauer told CBS News that the children were visiting their parent, who works at the restaurant as a night manager.

He said the children were not approved to be in that part of the premises by franchisee organisation management, adding that any work the children did was at the direction of their parent.

Mr Bauer also said that the company has worked to make sure the policy regarding children visiting parents and guardians at work is clear to all employees.

Furthermore, three franchisees that own more than 60 McDonald's locations in Kentucky, Indiana, Maryland and Ohio, were found to have "employed 305 children to work more than the legally permitted hours and perform tasks prohibited by law for young workers," the DoL said in a statement.

"Investigators from the department's Wage and Hour Division found two 10-year-old workers at a Louisville McDonald's restaurant among many violations of federal labor laws committed by three Kentucky McDonald's franchise operators," the DoL said in a news release.

"Investigators also determined two 10-year-old children were employed - but not paid - and sometimes worked as late as 2am."

It added that the three franchisees face a combined $212,754 (£169,788) in civil money penalties over the child labour violations.

Karen Garnett-Civils, the agency's wage and hour division district director, said: "Too often, employers fail to follow the child labour laws that protect young workers.

"Under no circumstances should there ever be a 10-year-old child working in a fast-food kitchen around hot grills, ovens and deep fryers."