Bunbury District Court Judge David MacLean said he "felt he had no choice" but to give a prison sentence to a former police officer.
The man, who is now in his 60s and has not been named to protect his victim, pleaded guilty to two counts of indecent dealing with a child under the age of 14. He was sentenced on July 25.
During the sentencing, prosecutor Alan Dungey said the offences occurred between 1975 and 1984.
The victim, who was related to the offender, was aged between four and nine years old at the time of the offence.
Judge MacLean said the offences occurred in the victim's home and bedroom while their mother was also in the house.
Prosecutor Dungey and judge both said how the offender touched the victim's genitals and placed the victim's hand on his genitals.
Judge MacLean described the actions as brazen and in gross breach of security and privacy, but accepted the actions were uncharacteristic.
During the sentencing, Judge MacLean said after the offence the man went on to join the police force and live an "unblemished" life and was a positive member of the community.
Judge MacLean said child abuse was a lifelong sentence for the victim and while the offender had been able to live a productive life for decades, the victim had suffered.
During the sentencing Judge MacLean read out parts of the victim impact statement which stated the "constant shame" and feeling of uncleanliness.
Judge MacLean read out to the court that the victim was now in their 40s and was still coming to terms with the "psychological demons" from the incident.
Judge MacLean said the impact statement showed that what had happened was a life sentence for the victim as they still struggled to establish themselves in the community.
Defence lawyer Michael Devlin asked the judge to give a suspended prison term.
The offender's psychological report showed that, due to his age, his admission of guilt and no other offences committed, he was a low risk to re-offend.
The man was given a 14-month prison term.
He will be eligible for parole in seven months.