In a case which could lead to the prison itself also facing legal action, Sabrina Bonner, 25, and the step-father, her partner Lionel Barthelemy, 31, are charged with multiple sexual assaults on the boy, who is now eight and in the care of social services.
Barthelemy admits raping the child in the visiting room at the Toul detention centre in eastern France in February 2010, but claims it was an isolated act initiated by Bonner.
She claims the prison incident and other rapes which she filmed were carried out under orders and the threat of violence from her partner, who allegedly listened to one assault from his prison cell on his mobile phone.
The trial, which is due to run for three days, will shed light on why prison staff failed to stop, or even notice, an act of such brutality taking place during what was supposed to be a supervised visit to a prisoner serving a sentence for domestic violence who had been diagnosed as having schizophrenic tendencies.
Prosecutors have indicated they will present evidence that black bin bags were used to cover the glass window of the door to the room in which the visit took place and that, as in other French prisons, this was standard practice to allow prisoners privacy with their partners during visits.
Yannick Pheulpin, the advocate representing the little boy, said he intended, at the end of the trial, to initiate legal proceedings aimed at holding the prison to account for the suffering of the child.
The prosecution alleges that Bonner blindfolded her son and made him kneel on a chair in front of Barthelemy, then held him by the arms as he was raped.
"Mrs Bonner made herself an accomplice to the crime by bringing her son to the visiting room twice on the same day: once in the morning when a first rape took place and a second time in the afternoon, knowing perfectly well that it was going to happen again," Pheulpin said.
Bonner has admitted carrying out multiple sexual assaults on her child during 2009 and 2010, filming them on her mobile phone and giving the phone's memory card to her imprisoned partner.
She gave herself up to police in May 2011, shortly before Barthelemy was due to be released from prison.
"She will not try to minimise her responsibility," her lawyer, Dominique Bergmann, said.
Bonner and Barthelemy face maximum prison terms of 20 years if convicted as charged.
This isn't the only scandal to have hit French prisons in recent years.
Back in February 2012, the director of a women's prison west of Paris was himself sent to jail for having an affair with one of the inmates, including having sex with her repeatedly in an IT room.
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During the spring, it took authorities six weeks to track down and capture Redoine Faid, a notorious French gangster who made a daring escape from jail, taking four prison guards hostage along the way.
In June, a judge took the rare step of ordering the release of a prisoner near Paris, due to the shocking condition of his cell. A hole in the ceiling left him cold and wet as wind and rain blew into the tiny living quarters.
There was widespread anger and embarrassment in August, when a legal technicality caused a French judge to pave the way for hundreds of convicted criminals to walk free from French prisons.