A mother and her partner have been handed lengthy prison terms by a court in southern Germany for the years-long sexual abuse of the woman's child, who was sold for sex with men on the dark web.

The 48-year-old mother was sentenced on Tuesday to 12 years and six months in jail, while her 39-year-old male partner was handed a 12-year term followed by "preventive detention," which could delay his release on the grounds that he poses a danger to the public.

The Freiburg court also ordered the two abusers to pay 42,500 euros ($A66,260) in damages to the boy and a young girl, another victim in the case.

The mother's partner, who already had a previous conviction of serious child abuse, confessed to the court in June that he was the "driving force" behind the crimes.

The 39-year-old admitted the 100-page charge sheet was accurate, except for a few minor details, saying that he had put pressure on his girlfriend to allow her son to be raped and abused by men they found online.

The mother confessed to the crimes during the trial but did not reveal her motive.

Prosecutors say it was partly due to her co-dependent relationship with her partner.

Six other offenders have been handed lengthy prison sentences as part of the case.

A Spanish national received a 10-year prison sentence on Monday after admitting that he had raped the boy on at least 15 occasions and had paid the mother and her partner more than 10,000 euros.