Andrew Fraser, 64, was treated as a respected member of the community in Dumfriesshire, Scotland.
It allowed him to groom and sexual abuse girls for more than 25 years before he was detected.
He assaulted one child while she slept and forced another, aged under 10, to use a sex toy, the Daily Record reports.
Fraser, who was remanded in custody at Glasgow High Court, faces a lengthy prison term when he is sentenced by Judge Lord Beckett at Edinburgh next month.
Detective Robin Ferguson of the public protection unit, said: “This is without doubt the largest sexual offences investigation involving one perpetrator that we have ever encountered in Dumfries and Galloway, possibly even Scotland.
"This investigation identified Fraser as having abused young children over decades.
"Sadly his crimes went unreported due to his style of grooming.
"He built relationships within the local community, with the parents and relatives of the victims, before proceeding to abuse them after being seen as a trustworthy person.
"He can only be described as a dangerous sexual predator and the damage he has caused is incalculable."
Fraser’s catalogue of abuse was uncovered when police found indecent photographs of youngsters on his computer.
Detectives were able to identify a number of the victims and Fraser was identified because of distinctive tattoos.
The predator initially faced 55 charges but admitted 17 of them against 12 girls between 1991 and 2015, when he appeared at court last Friday.
The court heard that Fraser was well known and parents trusted him to look after their children.
One of his victims was only six or seven when Fraser abused her at his home.
Another, aged 13, told police she had no memory of the abuse until she later saw the photographs.
The offences came to light after the police received information that he had a collection of indecent photographs of children.
Officers seized a computer, hard drives and a quantity of CDs and discs.
Prosecutor Stephen McCloy said: “A forensic examination of this computer equipment recovered still and moving images of children.
“These showed the sexual abuse of five of Fraser’s victims. There were also images which appeared to have been downloaded from the internet.”
A further search of Fraser’s home uncovered a pen drive and a Lenovo think pad laptop which contained indecent images of children.
Fraser, who is unemployed and on benefits, has previous convictions for theft and a racially aggravated breach of the peace.
Perverted babysitter abused 12 young girls for decades after gaining parent's trust JACKIE GRANT 10 AUG 2017