The Herald on Sunday understands the 13-year-old's father is also in custody.
The 13-year-old had been in the care of his maternal grandmother, who was praised by relatives as being a "golden lady".
"She has been the backbone of family making sure everything is going well for those kids," said a relative.
Family members described the boy's mother as having had a difficult path despite the support of her mother, now the caregiver for her children. She was a toddler and her younger brother was a baby when their father died.
Family said the grandmother has lived at the house for many years, caring for her daughter's children. The home is modest and secure, with a recently added full height security gate on the driveway.
As Kumar was being farewelled, family and friends gathered at the grandmother's home, where she had cared for many of her mokopuna.
A great-aunt of the 13-year-old said the boy and his siblings were supposed to be in the care of their maternal grandmother in Auckland while his mother was in prison.
"They were all fine when they were up here, we have heaps of kids up here but they are country kids and we bring them up differently, there is a lot of temptations down there for them to get into," she said.
Meanwhile, the 12-year-old's father spent his birthday in custody this month on violence charges.
The boy's mother was last before the courts about five years ago. She was convicted and sentenced on a dishonesty charge.
Kumar was farewelled by hundreds, including family friends, and Labour MPs Rajen Prasad and Phil Twyford, Auckland mayor Len Brown and deputy mayor Penny Hulse, at Ann's Funeral Home yesterday.
His daughter Sheenal told mourners: "We hope to be your children in our next lives."
"Everything you did in life was for us — and we are really glad to see us become the people you hoped we would become," she added on behalf of herself and brother Shivneel.
The Kumars' closeness and strong sense of family duty were evident in Sheenal's tribute.
"We missed out on the opportunity and the honour of taking care of you in our older years.
"It saddens us both that we won't have that opportunity and we won't be able to give you what you gave your parents," she added.
Arun gave Sheenal and her brother Shivneel "everything they wanted and more" and his wisdom and guidance would be sorely missed, the children said.
"Your honour and dignity will live through us," Sheenal added.
Despite the circumstances of Arun's death, master of ceremonies Sam Achary told mourners not to ask how or why the crime occurred, or to seek recrimination, but to comfort one another.
"The family is going through a difficult time today.
"Tomorrow it could be any one of us."
It's also understood two more youths could face charges in relation to the alleged burglary of Kumar's. Court documents showed the boys charged were alleged to have at least two co-offenders.
Extended family members spoke on online networks this week of disbelief and disgust at the stabbing.
Yet some friends in the same social network seemed to laugh off the incident.
Kumar, 57, died on Tuesday morning. His wife Anita was in the shop during the attack and remained by his side as the ambulance arrived.
The youths remain in custody until they appear in court later this month.
