The pair will retain name suppression until at least the end of next month when defence lawyers will apply for permanent non-publication orders.
Outside court, the family of Mr Kumar spoke of their disgust at the outcome.
"The heart and soul of our family was taken away," the victim's son Shivneel Kumar said.
CCTV footage of the horrific incident, from inside the Railside Dairy on June 10 last year, was played regularly during the three-week trial.
"We endured countless reruns of my dad's last moments. We tolerated all this to obtain justice for my dad."
Shivneel, supported by his sobbing mother Anita, said they now felt that had been for nothing. "We are now living in a society where kids are on drugs, roaming the streets with weapons at the ready," he said.
There was never any dispute during the trial that the 14-year-old caused the fatal wound to Mr Kumar when he stabbed him in the neck, or that his co-defendant was standing outside the shop with a metal pole.
But defence counsel spent much time detailing the boys' backgrounds and the psychologically debilitating effects they had on them.
The court heard evidence that the older defendant suffered a severe head injury after being hit by a car six years ago.
The lack of rehabilitation and an addiction to synthetic cannabis resulted in a reduced mental capacity, particularly in "complex" situations.
His lawyer, Maria Pecotic, argued that such a situation arose in the dairy when Mrs Kumar entered with a phone to call police.
The jury also heard details of the 13-year-old's troubled upbringing.
His lawyer, David Niven, said his client had a brain "like Swiss cheese" after his mother smoked methamphetamine and consumed alcohol while pregnant with him.
He had taken drugs and witnessed regular domestic violence, which contributed to his lower IQ and behavioural disorder.
Mr Niven told the court the boys' original idea was to break into a nearby shoe shop. The change of plan to hit the dairy came at the eleventh hour and they would have had less than three minutes to plan the theft.
The Crown said the armed robbery might not have been well planned but each of the boys wanted money and had weapons to achieve their goal.
Prosecutor Kieran Raftery said the older boy had the knife concealed and "at the ready" and they must have known there was a reasonable possibility of violence, making him guilty of murder.
However, the jury disagreed.
The 14-year-old was convicted of manslaughter and remanded in Child, Youth and Family (CYF) care by Justice Graham Lang.
He is one of about 10 children. A source close to the family said most of the children had been in and out of CYF care. He will be sentenced next month.
Dad behind bars
The father of the 13-year-old boy acquitted of manslaughter in the Arun Kumar trial is in prison awaiting sentencing over a home invasion in October 2013, near where Mr Kumar bled to death.
He and six other men, dressed in Mongrel Mob regalia, kicked in the door of the house. A female resident was stripped of her jewellery, a man had a pot plant smashed in his face, and another woman had petrol poured over her and was told: "I'm going to make an example of you, b****".
Six men will be sentenced at the end of August.
