Speaking to reporters hours after the accident, which took place close to the village of Courteranges near Troyes in central France, local prosecutor Alex Perrin revised an earlier toll that had said five children and their driver died in the crash.
"There was an extremely brutal impact and six people were killed -- the driver, four children and a 20-year-old man," he said, adding the children were aged 11 to 13.
At least three other passengers were badly injured in the accident yesterday, but their condition was not critical while the truck driver was "in shock", Perrin said.
French Interior Minister Bernard Cazeneuve and Transport Minister Frederic Cuvillier visited the site of the crash earlier yesterday.
Perrin said a probe into the accident would seek to determine the exact cause of the crash, adding that the road was well maintained, and visibility and the weather at the time were good.