Alexander Hunter was found guilty of seven charges on August 21 following a trial at the High Court in Edinburgh.
The 66-year-old, from Kelty, Fife, raped and abused his victims on various occasions between 2005 and 2016.
The children were aged seven and eight when Hunter began targeting them.
On September 19, at the High Court in Edinburgh, he was given a 12-year extended sentence with nine years in custody.
His name has been added to the sex offenders register indefinitely.
Non-harassment orders, banning Hunter from contacting or attempting to contact his victims, were also granted for an indefinite period.
Scotland’s Procurator Fiscal for High Court Sexual Offences, Katrina Parkes, said: “Alexander Hunter posed a danger to young girls by exploiting his position of trust in the worst possible way.
“The trauma these victims suffered was at the forefront of our minds during this prosecution. It is thanks to the bravery of those affected by Hunter’s offending that he has now been brought to justice.
“We would encourage anyone affected by similar offending to come forward - regardless of how much time has passed. You will be listened to and supported as we seek to secure justice using all the tools at our disposal.”
Police Scotland Detective Constable Emma Moyes, from Fife’s Public Protection Unit, added “Hunter showed utter disregard for the impact these crimes would have on his victims and their families.
“I know it can be incredibly difficult for people to report incidents of this nature and we know how challenging this case has been for those involved.
“We are wholly committed to investigating child abuse, regardless of when the crimes took place, and will continue to work closely with our partner agencies to ensure offenders are brought to justice.”