If you can't trust an assistant school principal, who can you trust?

That was the question an Illawarra mother found herself asking a Sydney court to consider when determining an appropriate punishment for Mark Forbes, the man responsible for molesting her two young daughters.

The one-time Albion Park Public School assistant principal was found guilty in June of sexually assaulting the young girls while they were staying the night at his house on March 27 last year.

It is understood the girls were not students at the school, but the mother was aware of Forbes's profession.

Facing a district court sitting on Wednesday, Forbes sat quietly in the dock while the woman attempted to explain the toll his actions had taken on her family.

"She felt disempowered and unsure of herself ... she was visibly distressed and crying," the woman said, speaking of her older daughter's reaction on the night.

The woman said the incident had led the family to reduce its level of socialising, leaving them feeling isolated.

"The incident has shaken our sense of trust in people," she said.

"If you can't trust an assistant primary school principal, who can you trust with your children?"

She said her younger daughter, who was subjected to the more serious case of violation of the two girls, had been deeply affected by the assault.

"She lost her childhood innocence that night," the woman said.

"Her world was thrown into disarray and confusion. She has became anxious and fearful as a result."

The woman said she was torn between detailing to the court her daughters' distress and maintaining their sense of privacy, saying her older daughter felt embarrassed by the incident.

Meantime, Crown prosecutor Kate Ratcliffe argued Forbes's offending constituted a breach of trust on his part, as the children had been in the care of him and his wife at the time.

Defence lawyer Susan Oliver described the acts as "opportunistic" rather than planned, noting the short duration of the assaults.

She said Forbes, 54, had lost his job and his marriage because of his conduct.

Judge Mark Marien will hand down his sentence on October 21.