The girls' mother said "her whole world crumbled" when she found out about the offending, which she described as "every parent's worst nightmare".

"Her initial feelings of shock and disbelief were followed by months and years of feeling angry, betrayed, shame, guilt, disgust, hurt, embarrassment, humiliation, hate and extreme sadness," Judge Susan Pullen told the County Court recently as she sentenced the now 78-year-old.

The Wangaratta man targeted the primary school aged girls from 2010 to 2014.

Much of his offending involved touching and rubbing the girls over and under their clothing.

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Some of the offences occurred at his home or in Melbourne.

The girls' father confronted him after learning of what had occurred, but the man denied the allegations.

"He constantly asked himself why you did this to his children," Judge Pullen said.

"He trusted you implicitly.

"You had destroyed the very foundation of the family, love and trust, for your own selfish needs.

"He felt nothing but angst and emptiness.

"He said it was not fair you had continued to live a normal life and act as if nothing had happened, whilst his daughters live with the pain of your offending throughout their childhood."

The offender had claimed to police he may have "incidentally" touched one of the girls, and suggested the other made up the allegations because he had told her off for staying up late.

"An aggravating feature of your offending was that it was a breach of trust, not only of (the girls), but also their parents who trusted their daughters, your granddaughters, in your care," the judge said.

The man had continued to target one of the girls despite her saying she was uncomfortable and leaving the room.

Judge Pullen noted the victims had suffered considerably as a result.

A maximum term of three years and three months was imposed.

He will be a registered sex offender for life.