The woman, 40, who cannot be named, gave birth to a daughter in May last year when her victim was 14.

The victim's mother, who was a close friend of the offender, told the County Court on Wednesday that her son now sees the baby girl as his "little princess" and "shining light" but his life had been destroyed.

She revealed how she had been by her close friend's side when she gave birth in hospital unaware that the child was her son's.

The victim's mother said the offender initially denied that the boy was the father and declared that that would be "disgusting". The father's name is not listed on the child's birth certificate.

Crown prosecutor Joanne Piggott said the offender had been married and living in Melbourne's outer south-east with her three children when her daughter became friends with the victim when they were both 11.

Ms Piggott said the offender was jealous of her daughter's relationship with the victim and began taking him to and from school when she was having marital problems.

The offender, who was often paid by the victim's mother to babysit him while she was at work, first had unprotected sex with the boy when he was 12 and in year 7 and she was 36.

The boy at the time was vulnerable, depressed and in a bad place after his parents had separated.

Ms Piggott said it was the boy's first sexual experience and he later told police they had sex more than 20 times over the next two years, including in her car and when camping in the back yard.

The offender's daughter told police she always thought there was something going on between her mother and the boy sexually.

The offender, who pleaded guilty to one count of persistent sexual abuse of a child under 16 between April 2012 and May 2014, faces a maximum sentence of 25 years' jail.

The victim's mother told the court her son's childhood had been taken away and his life destroyed by the offender and he believed he was an adult.

She said the boy thought he was older than he was and should be treated that way so he had no respect for authority.

The mother said her son was struggling to cope with the situation, cannot go back to school and had self-harmed.

The boy had lost the chance to fall in love and have sex for the first time with someone special.

Defence lawyer Rebekah Haylock said the offender was ashamed, embarrassed and guilt-ridden over what she had done.

The offender had told a psychologist how she was sexually naive and did not realise the boy was old enough to father a child.

Ms Haylock said the offender had been missing her husband when he was away for work, was depressed and lacked self-esteem when she crossed the boundary from platonic carer to sexual partner.

The offender claimed she was attracted to the boy because he was sexually maturing and she was drawn to him. She denied being attracted to children.

"It's a tragic situation, absolutely tragic," the judge, who cannot be named, said.

"She has behaved, in my view, absolutely appallingly. It's dreadful."

The judge said the offender had to be jailed to send a message to the community that people cannot have sexual relationships with children.

Ms Piggott said the offender was guilty of an extreme abuse of trust and had ruined many lives to fulfil her own needs and desires.

"There has been absolutely no regard for the damage caused to the child," the prosecutor said.

The victim and his family would feel the effects of the woman's offending for the rest of their lives.

The offender was remanded to be sentenced next month.