A teen motorist who killed a four-year-old boy when his modified car mounted a footpath in suburban Christchurch in May has avoided a jail sentence.

Judge Phillip Moran sentenced Ashley David Austin, 18, who admitted dangerous driving causing the death of Nayan Woods, 4, as well as injury to his brother Jacob, 6, and mother Emma Woods, to six months' community detention, 200 hours' community work and disqualified him from driving for three years.

In a remarkable display of forgiveness, Emma Woods embraced a sobbing Austin outside Christchurch District Court after the sentencing this afternoon.

"It's easy to raise voices in anger and condemn others for their mistakes," Mrs Woods said outside court. "Unfortunately, human nature means mistakes are often made.

"Often what separates one mistake from another is the consequence that results. In this case, the consequence was immense and the devastation that has followed is inescapable.

"Although the effects of this mistake cannot be undone or made right, we recognise that Ash [Ashley Austin] has done everything he can to support us.

"We do not believe he can be punished any more severely than by having the guilt of this accident on his conscience.

"We hope the community can stand in support of him as he seeks atonement," she said.

Austin was too upset to read his own written statement outside the court.

But in his statement, read by his mother, he thanked Mrs Woods and her husband Duncan for the "compassion and empathy they have shown towards me despite the pain and suffering that this tragedy has caused.

"It has been an honour to spend time with Emma and Duncan and to learn about Nayan and Jacob. The Woods family is an amazing family," said the statement.

"I know that no apology or action can take away the loss, hurt and suffering I have caused. But I am truly sorry and extend my biggest and deepest apologies to all of Nayan and Jacob's family and their friends."

During sentencing today, Judge Moran told Austin that "the parents of the little boy that you killed forgive you. You are clearly a young man of good character. You are not a boy racer."

When he appeared in Christchurch District Court in July, Austin admitted dangerous driving causing the death of Nayan and injury to his brother and mother as they walked on the Linwood Avenue footpath on May 21.

A vehicle inspector found the Nissan Silvia Austin was driving had been modified to the point where it was "only suitable for a controlled environment like a race track".

The court heard Austin was driving from work in the Nissan, registered to his mother, when he turned into multi-lane Linwood Avenue, and accelerated in first gear in what witnesses described as a "controlled drift", but which Austin said was not deliberate.

When he changed into second gear while still accelerating, the car swung out at the rear and Austin lost control.

The car mounted the grass verge, drove across the footpath and ran head-on into a small fence.

It then rotated about 180 degrees and struck Mrs Woods and her two sons, coming to coming to rest on top of Jacob.

Nayan suffered the worst injuries, and Mrs Woods, Austin and police and paramedics tried in vain to resuscitate him. Jacob had multiple injuries, including fractures, and Mrs Woods had extensive bruising.

Austin later told police he felt he was too quick with the clutch while changing from first to second gear in high revs, and this was not his "everyday type of driving".

A vehicle inspector's report showed the car had lowered suspension with stronger springs to enable the car to slide easier, and the hand brake did not work on the rear wheels.

The modifications could have contributed to the cause of the crash.

Speaking after Austin admitted the charges in July, Nayan's mother Emma Woods said she was relieved by the guilty pleas but did not want believe her son's killer was a bad person and did not want him jailed.

"I guess he made a mistake that had pretty horrendous consequences, but that doesn't make him a bad person," Mrs Woods said.

"And he's done a lot since the accident to attempt to make amends or try to support us.

"I don't think somebody like him belongs in jail. I don't think he's going to learn anything from being in there. The mistake he made, he's not going to do it again."