These are the words of a grandmother to her seven-year-old grandson who died after months of extreme abuse and neglect at the hands of his mother and her boyfriend.

The boy's 27-year-old mother and her boyfriend first claimed the boy's fatal injuries were caused by a fall from a pogo stick in Sydney's south in May 2013.

But the mother has pleaded guilty to manslaughter and the boyfriend, a Christian hip-hop producer, will face a murder trial in October.

At the mother's sentencing hearing in the NSW Supreme Court on Tuesday, the boy's grandmother and grandfather told of their grief and the torment felt by the child's siblings.

The grandmother made loving tributes to the boy whose photographs adorn her house.

"You left us peaceful memories. Although we cannot see you, you are always at our side," she said.

"You did not go alone. Part of us went with you."

She said the boy's siblings were struggling to cope with memories of what happened to him and were coming to grips with losing both their brother and mother.

"We have picked up the pieces to get [the siblings] to be able to put this behind them and one day lead productive lives, learn to trust again and develop healthy relationships," she said.

"We want them to start sleeping without the nightmares, stop curling up when voices are raised, stop hitting others and not be afraid if they've done something wrong."

The grandmother angrily addressed the boy's mother, her daughter-in-law, a slight young woman, who cried and wiped away tears as she sat in the dock.

"You brought them into this world and as a parent … you failed to protect them.

"[You're] the one person he would have trusted most. He had no one or nowhere to turn for safety."

She said the mother's name is never mentioned in front of the children.

"We can't wait for the sentencing. We hope you get what you deserve. The system needs to be stepped up – there's a lot of children who need help."

The woman's father remembered the boy's cheeky smile, his love of parties and the big hugs he gave his siblings.

But one memory that sticks with him is seeing the boy's body at the morgue.

"He looked like he'd been run over by a truck," he said.

"It made me cry to see [him] like this. I have never seen my daughter since."

According to a statement of facts tendered to the local court in March the mother and the three children moved in with the man within a week of meeting him in January 2013.

Police say her care of the children significantly diminished once she moved into his recording studio which did not have a toilet or bathroom and was regularly filled with people smoking cannabis.

The couple made the children use a bucket for a toilet and put them in nappies at night, despite all three being toilet trained, so they didn't have to use the communal toilets in the complex in order to hide their presence. Their bedding was covered in faeces and urine.

The man instigated a punishment regime based on the concept of "Spartans and soldiers" from the action movie 300, which included "boot camp" style exercise and vicious beatings.

Special punishment was reserved for the seven-year-old boy who had a moderate intellectual disability and was described by his teacher as a timid, softly spoken child.

The mother's hearing continues before Justice Ian Harrison.