Jimmy Martin Brito lost control of his Subaru WRX while street racing and slammed into a tree in the suburb of Monterey, in Sydney's south, in August 2023.
The passengers of the vehicle, two brothers aged nine and 10, died in hospital soon after the crash, while a nine-year-old girl survived with minor injuries.
The boys' father and stepmother told a sentence hearing on Wednesday they gasped when they saw the crumpled vehicle at the site of the crash, which occurred only a few streets from their home.
"It was clear why they had not survived," the woman said.
A short time earlier, the couple had to identify the bodies of the boys in hospital, still dressed in their school uniforms.
Police said Brito was driving more than 45km/h over the local speed limit of 60 before the crash.
The now 34-year-old, who suffered serious but non-life threatening injuries, pleaded guilty to two counts of manslaughter and one count of causing bodily harm by misconduct in charge of a motor vehicle.
An Audi allegedly involved in the street race was also seized following the incident and its driver was charged.
Mobile phone footage from inside the Subaru just prior to the crash was also shown during the sentence hearing in Sydney's Downing Centre District Court.
In the footage, the boys can be heard apparently cheering as the speeding vehicle changes gears.
Crown prosecutor Gabrielle Steedman asked the court not to give weight to submissions the children's screams of "glee or excitement" alleviated Brito's responsibility.
Judge John Pickering agreed that whether the children were enjoying the street race was not relevant to Brito's level of responsibility as the only adult present.
"A child might be excited not wearing a seatbelt too," he said.
"They're just not in a position to understand the risk and seriousness of what is occurring."
Appearing in court from custody via a video link, Brito looked distressed throughout the hearing and at one stage covered his face and appeared to sob uncontrollably.
Defence barrister Taran Ramrakha argued Brito's choice to engage in a street race could be linked to his mental health issues, including learning difficulties and undiagnosed ADHD, as well as being dropped on his head when he was young.
"It's something that in my decision can only sensibly be explained by reference to his mental health issues," he said.
"It's a pretty stupid thing to do otherwise."
Judge Pickering firmly pushed back against the explanation, describing it potentially as "clutching at straws".
"Humans sometimes just behave poorly; even more so in the circumstance of driving," he said.
"He wasn't someone who necessarily had a very good driving record.
"He's someone that enjoys speeding a car and enjoyed doing the racing, enjoyed a bit of showing off."
A sentence date has been set for March 7.
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