Children as young as 12 have been physically abused, detained and forcibly returned to Italy by French border guards in a breach of international standards, Oxfam has claimed, reports Sky News.
A report from the charity says children complained about being "physically and verbally abused, and detained overnight in cells without food, water or blankets and with no access to an official guardian" - all contrary to French and EU law.
It says French police routinely stop unaccompanied children and put them on trains back to Italy after altering their paperwork to make them appear older, or to make it appear that they want to be sent back.
They claim around 16,500 refugees and migrants, a quarter of them unaccompanied children, passed through Ventimiglia, a small town about seven kilometres (four miles) from the French border, in the nine months to April.
Allegations of border guards cutting the soles of the shoes of child migrants and stealing their phone SIM cards have been made.
"In one case, a very young Eritrean girl was forced to walk back across the border along a road with no pavement carrying her 40-day-old baby," the report said.
The "Nowhere but Out" report comes after French President Emmanuel Macron criticised Italy over its rejection of a migrant ship carrying 629 people rescued from the Mediterranean.
The Aquarius ship and two other boats headed to Spain after being stranded for days off the coast of Italy.
Italian Prime Minister Giuseppe Conte's office hit back, saying: "Italy cannot accept hypocritical lessons from countries that on the topic of immigration have always preferred to turn their heads."
Nearly 39,900 migrants were refused entry to France at the Italian border from January to August 2017, according to official data.
"Since France tightened border controls in 2015, thousands of people have been stranded in Ventimiglia without sufficient assistance or access to basic services," Oxfam said.
"Hundreds of refugees and other migrants sleep rough under a flyover, with no access to drinkable water, shelter or heating."