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 7km (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.”
Many of the migrants are fleeing persecution and war in countries such as Sudan, Eritrea, Syria and Afghanistan.
“French police officers are not upholding international standards. They taunt children and mistreat them… Some children have had the soles of their shoes cut off before being sent back to Italy,” said Chiara Romagno, Oxfam’s OpenEurope project leader in Ventimiglia.
Oxfam said France should immediately stop and adequately sanction the “illegal practices of the French police at the French-Italian border”, ensure the right to seek asylum for all foreign children in French territory and take care of them according to the provisions of national, European and other international laws.
An EU scheme to distribute migrants equitably around the bloc has failed.
Some central European countries are flatly refusing quotas, while others, such as France, are falling far short of their allocated target.
Oxfam spokesperson Kai Tabacek told Sky News that the UK could be doing more.
He said: “The UK could be doing a lot more to resettle refugees and vulnerable migrants.
“One small but constructive step would be to make it easier for refugees to reunite with family members who are already in the UK.
“Currently this is allowed but only in very specific circumstances.
“We call on the UK and all European countries to reform the practises around refugee family reunion to ensure families are not kept apart.”