A SINGLE mother who had three children with a teenage neighbour during a five-year relationship that began when he was 13 has avoided jail due to "exceptional circumstances".

County Court Judge Chris O’Neill said that the hardship that would be suffered by the now- married woman's children if she was imprisoned, as well as her reduced moral culpability due to cognitive deficits, meant her jail term would be wholly suspended.

The woman — who cannot be identified — pleaded guilty to maintaining a sexual relationship with a child under 16, which carries a maximum penalty of 25 years’ imprisonment.

The judge said the woman, who was in her early 20s when she began a sexual relationship with the 13-year-old, believed she was in love with the boy, whom she gravitated towards because they functioned at similar cognitive levels.

He said she provided a liberal environment for the teen, which included access to drugs, alcohol and freedom, but did not groom him or act in a predatory nature.

"Rather, you were simply attempting to endear yourself to (the boy) as you felt genuine affection towards someone with whom you could relate," Judge O'Neill said.

The teen became a father at 15, and again at 16 and 18, which the judge said would have had a profound impact on him.

"He had to deal with the pressures of fatherhood at far too early an age," Judge O'Neill said.

He said the boy’s mother, police and the Department of Human Services all vainly tried to intervene, but both the boy and the woman denied there was any inappropriate relationship.

The court previously heard the boy noted about 26 occasions on which the couple had sex in his year 7 school diary.

It wasn't until 2011, in the context of a custody battle over their three children, that he reported the relationship to police.

The judge said the woman — now aged in her 30s and married with a further two children — spent every waking moment caring for her children, including one who suffered from a mild disability.

He said even if one or two of her youngest children were allowed in prison with her, the children would suffer significant hardship were she not available to them.

Judge O'Neill also said her cognitive deficits meant she had a reduced understanding of the risks involved with the illicit affair and less able to make the appropriate decisions.

The woman had good prospects for rehabilitation and was not sexually attracted to children, the judge said.

He said normally this offending would warrant a significant jail term, but due to the exceptional circumstances, he imposed a three-year term of imprisonment, wholly suspended for three years.

The woman will be a registered sex offender for life.

The identification of the woman and her children was previously prohibited in order to protect their safety, owing to “concerning and serious comments” on a public Facebook page set up by the victim.