The attacker, who was eventually given probation, waved to the girl in her school playground the day after the attack, delegates heard.
Activist Anne McLaughlin told the conference: "Imagine being that child - imagine being that mother."
She told of the case during a debate in which the conference called for the Scottish Government to set up a sentencing commission to review sentencing options for sexual offences against children.
McLaughlin told the conference: "The little girl kept it to herself for years because the tradesman told her if she told anyone he would get her little sisters next."
Three years later the little girl broke down when her mother called in another tradesman.
The case came to court when three other children came forward to say they had been victims of the man, who denied the allegations until "the very last minute".
He was convicted of lewd and libidinous behaviour against four children but was put on probation as a first offender, she said.
"I know prison is not the only answer, but what message are we sending out to this little girl?" said McLaughlin.
"Many people are unhappy and have many questions about the consistency and effectiveness of dealing with child sex offenders - not least the children themselves."