Police officers said that this case was like nothing they had ever seen. (Representational)

A German court handed down lengthy jail sentences Tuesday to a couple for repeatedly sexually abusing their young son and selling him to paedophiles online, in a shocking case that raised pointed questions about the competence of child protection services.

Following a criminal probe that even seasoned investigators said pushed them to the emotional limit, the regional court in Freiburg sentenced the boy's mother, Berrin Taha, to 12 and a half years in prison.

Her partner Christian Lais, the boy's stepfather whom he called "papa", received a 12-year sentence followed by preventive detention.

Lais, 39, was allowed to live in their home despite a previous conviction for child abuse and a court order to stay away from minors.

The judges found that the unemployed couple had sexually assaulted the boy, now 10, numerous times and pimped him on the so-called darknet between May 2015 and August 2017.

They were convicted of rape, aggravated sexual assault of children, forced prostitution and distribution of child pornography, judge Stefan Buergelin said.

The couple was also ordered to pay a total of 42,500 euros ($49,200) in damages to the boy and another victim, a girl who was temporarily in their care.

Taha, 48, said through her lawyer that she would not appeal, in order to spare her son an even longer legal ordeal.

Vague' abuse suspicions dismissed

Their sentences fell short of what prosectors had demanded -- 14 years and six months for Taha and 13 years and six months for Lais.

Even experienced police officers dealing with child abuse cases said the case was like nothing they had ever seen.

Local judicial and child services officials came in for massive criticism for having interviewed the mother after concerns about the boy's welfare arose months before the couple's arrest but failing to get his own account.

Key information about the case was not shared among authorities, and warnings from the boy's school about his wellbeing were dismissed by youth services as "vague".

On Monday, the same court sentenced a Spanish paedophile, Javier Gonzalez Diaz, to 10 years' prison for repeatedly sexually abusing the young boy after paying the parents.

The 33-year-old was also ordered to pay 18,000 euros to the young victim.

The case came to light following an anonymous tip-off last September, and led to the arrest of eight people who have been charged with belonging to an online paedophile ring.

Police said they have another 20 suspected perpetrators in their sights.

Owe it to this child

Lais admitted on the witness stand to the assaults on the boy.

He also made serious accusations against the child's mother, who confessed to the crimes but remained largely silent about any possible motive during the trial.

In his verdict, Buergelin said Taha went along with the abuse at first so she would not lose Lais as a partner and later "for financial motives".

They charged several thousand euros (dollars) for each instance of abuse, filmed the acts and posted them on the darknet.

The boy is in foster care and did not have to testify during the trial. His lawyer told reporters Tuesday that he was doing well "under the circumstances".

The German government's ombudsman for child sexual abuse issues, Joahnnes-Wilhelm Roerig, said the case had "laid bare a series of errors of judgement and shortcomings" by local, state and federal authorities.

"We owe it to this child to draw the right conclusions," he said, including improving cooperation between services, staffing and training.

He said in a statement that an official report on what went wrong in the case, compiled by judges and youth office staff, would be presented in September and include a list of recommendations for future policy.

Roerig also noted an official bias presuming that a mother would never sexually assault her own child, despite the fact that an estimated 10 to 20 percent of abusers are female.