His father, Miroa Simpkins, was ordered to pay one victim $250 in emotional harm reparation.
The incident happened in January last year after one of the boys stole a pair of shoes from outside their Ngongotaha home.
Ngarimu Simpkins pinned one on the ground and punched him in the head more than once.
He also kicked or punched the boy in the stomach, winding him, and later punched one of the boys in the face, knocking him to the ground.
Two boys were taken to hospital where they were treated for injuries. A third boy was uninjured but shaken.
Lawyer Max Simpkins said the men recognised they overreacted and that Ngarimu Simpkins' response was borne out of his conditioning to protect his family and family home.
Judge Maree Mackenzie said he had overstepped the line considerably and "it would be a world of carnage if everyone responded in that way".
"This was a vigilante response when you should have contacted the police. It was not your job to punish the boys."
In a tearful submission to the court, the grandmother of one of the victims spoke about the impact of the incident on her grandson.
"He was an innocent bystander who has been totally affected by this issue."
She said he "frequently visualises the attack and has isolated himself".
"You destroyed a beautiful boy."
Judge Mackenzie acknowledged the "real and palpable" effect the assault had on the victims but also the steps Miroa and Ngarimu Simpkins had taken to rectify their wrongs.
A restorative justice meeting was held with the victim's families, Ngarimu Simpkins completed an anger management course and a hui was held at a marae to apologise to the community.
Judge Mackenzie also made a point of mentioning the "glowing" references provided by Rotorua Boys' High School principal Chris Grinter and deputy principal Bruce Buckton.
"It is clear you [Ngarimu Simpkins] are held in high regard and on the face of it have the trust, faith and support of senior staff at the school."
Outside court, the grandmother said she was pleased with the end result and hoped the ending would provide some closure for the three boys.
"I welcomed the conviction and take my hat off to the judge who was very thorough and fair to both sides.
"No amount of money will fix what they have done but I am hoping now this is over we can take him under our wing and get him back on track.
"He was such a well adjusted kid before all this."
The mother of one of the other victims told the Rotorua Daily Post she was glad the process was over and the family was able to move on.
"I accept Ngarimu is remorseful and believe his apology was genuine but it sucks it happened in the first place. I know my son has accepted their apologies but I don't know if I can fully forgive someone who manhandled my child."
Mr Grinter said the school would not be making any comment as they had not yet met with Ngarimu Simpkins.
Charges:
Ngarimu Simpkins: 2x assault on a child, 1x common assault, 1x assault with intent to injure
Miroa Simpkins: 1x assault on a child under 14, 1x assault with intent to injure.
