Derek Lamond was found guilty of 10 charges on August 7 following a trial at the High Court in Edinburgh.
The 59-year-old targeted his victims at various addresses in Fife as well as during car journeys elsewhere in Scotland.
Lamond provided the two children with cigarettes, alcohol and money during the abuse.
He also offered them money to expose themselves to him.
He was found guilty of assaulting and raping one child and attempting to rape another between the mid-90s and 2016.
Lamond was also found guilty of multiple sex offence charges involving his victims.
On September 4, at the High Court in Edinburgh, Lamond was given an extended sentence of 15 years, with 12 years in custody.
His name has been added to the sex offenders register indefinitely.
Scotland’s Procurator Fiscal for High Court Sexual Offending, Katrina Parkes said:
“Derek Lamond manipulated and exploited these young children in an abhorrent campaign of abuse.
“His victims have shown great courage and strength by coming forward and reporting him to the authorities.
“Lamond will now have to face the consequences of his actions following this prosecution and conviction.
“I would encourage all victims of similar offending – no matter how long ago it may have occurred – to come forward, report it and seek help.
“Scotland’s prosecution service is committed to securing justice for victims of child sexual abuse.”