The father of a seven-year-old boy who died after prolonged abuse says authorities should be ashamed for not protecting his son.

The child was found dead in his home in the southern Sydney suburb of Oatley in May after he reportedly fell off a pogo stick.

It was later found the boy, who cannot be named for legal reasons, died from head injuries.

The boy's father has been living in New Zealand without his three children for the past three years after he separated from his partner.

Now, the Family Court is letting him take the youngest two children home.

"They're meant to be there to protect your children, but the system here's failed, failed so much, in so many cases," he told the ABC.

"The main thing is my two young ones are safe now, that's the main thing. They're happy with me and my family and I just hope we can get on with our lives and put this nightmare behind us."

Last week, the 29-year-old partner of the boy's mother was refused bail after being charged with 25 child abuse offences.

Court documents allege the seven-year-old boy was forced to sit outside alone in his underwear as punishment for soiling his pants, repeatedly assaulted with a spatula, thrown across the inside of a tent and forced to run along a beach until he fell over with exhaustion.

There is also video of the abuse, including footage of the man encouraging the boy's younger siblings to assault him.

The Department of Community Services knew about the alleged abuse but the children were not removed.

It says investigations into the case are continuing.

The boy's grandmother, who flew to Australia with her son to collect her two remaining grandchildren, says more must be done to protect children.

"We all say he's in a happier place but he shouldn't be in that place - a happy place is here with us," she said.

"Hire more people to protect our babies, protect our loved ones, they are always the innocent parties in this world."

The NSW Government says it is filling all case worker vacancies, but the Opposition says it is not happening quickly enough.

"What this underscores is that every position in a department must be filled," said Linda Burney, the Opposition spokeswoman for family and community services.

"It must be filled with permanent staff and importantly with staff that are supported and trained properly."