The boy's mother, who cannot be named for legal reasons, drowned her son in the river at Moama in March 2017, and attempted to drown her older son.
She was later found not guilty of murder on mental health grounds, after a Supreme Court judge found she suffered from a series of psychiatric conditions and held a delusional belief that she had to drown her sons in order to protect them.
The boys' grandmother appeared reluctant when she was called to give evidence in the Coroner's Court at Deniliquin, and let out a heavy sigh as she sat down.
She told the court how the mother of the children, her daughter, wrote letters to her while she was in jail in 2016 and 2017, one of which outlined the mother's plans to take the children to Queensland.
The grandmother said she took that letter to the Department of Family and Community Services.
She told the court she was extremely concerned that no orders were in place that could stop her daughter from taking the boys.
"I was really frightened for the boys," she said. "There was nothing in place at the time and I did ask about [what] would happen if she takes them to Queensland,” she said.
The grandmother recalled the boys' mother calling Deniliquin a "dump" and saying she was going to take her babies as far away from that town and her as possible.
When asked if she was against the move because it would mean the boys would be far away from her and she wouldn’t be able to see them, the grandmother tearfully replied "no".
In the days following the drowning incident in 2017, NSW Premier Gladys Berejiklian ordered an investigation into how a number of reviews by government agencies handled the family's case.
A spokesperson for the Department of Communities and Justice confirmed those reviews had been completed and handed to the State Coroner for consideration.
In her opening address to the inquest the counsel assisting state coroner Teresa O'Sullivan, senior counsel Kristina Stern, told the court the inquest would look into what warnings government agencies were given about the mother's behaviour in the years leading up to the tragic incident.
Ms Stern outlined how, in 2016, the children were assessed by caseworkers from Family and Community Services as being "safe" and at "low risk" because they were in the care of the grandmother.
The matter was closed despite the children being considered at a "high risk" when in the company of their mother, who had the legal right to care for them.
Ms Stern told the court the departmental reviews into potential "systemic failures" would be discussed in the final days of the inquiry.
Ms Stern outlined how on the day of the incident the grandmother reported that the boys were missing after their mother took them swimming and did not return.
Under questioning, Detective Sergeant Trent Swinton, who was with the Murray River Police District at the time, told the court that the police response in hindsight "could've been more prompt".
The case was marked as the least serious of three alert levels, with officers told to "keep a lookout for" the children, rather than the case being treated as a more serious "concern for welfare" or "missing person" situation.
When presented with an email exchange between two constables, where a car was tasked to follow up on the grandmother’s concern on the morning of March 2, 2017, Sergeant Swinton said the situation appeared more serious.
The inquest, which has been set down for two weeks, continues.
— Mahnaz Angury and Lauren Pezet, ABC Riverina
Inquest to probe 'systemic failures' of welfare agencies prior to drowning of boy - ABC News
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