A drunk and drugged driver who killed four children in a crash in Sydney's west has had his sentence slashed by eight years on appeal.

In February 2020, Samuel Davidson lost control of his ute and mounted a kerb in Oatlands and hit a group of seven children.

Siblings Antony, 13, Angelina, 12, and Sienna Abdallah, 9, and their cousin, Veronique Sakr, 11, died at the scene.

Three other children were injured.

In April last year, Davidson was sentenced to 28 years imprisonment, with a non-parole period of 21 years.

He then appealed his sentence, arguing it was manifestly excessive, which was accepted by the Court of Criminal Appeal.

In its judgement published today, the court reduced Davidson's sentence to 20 years, with a non-parole period of 15 years.

The court ordered it impose the eight-year sentence reduction suggested by Justice Natalie Adams, while the more senior judge, Justice Paul Brereton, proposed a reduction of 12 years.

A third judge, Justice Christine Adamson, dissented and dismissed the appeal.

In the judgement, Justice Paul Brereton said there had to be regard for '"the violation of each victim, but also to the circumstance that they were the result of but a single course of criminal conduct."

"To produce an aggregate sentence of 28 years was grossly discordant with prevailing sentencing practices."

"A sentence for an offence of criminal negligence which leaves a 29-year-old man of prior good character with good prospects of rehabilitation no prospect of release until he is 50 is indeed crushing, and not proportionate to the totality of his criminality."

Justice Brereton said the indicative sentences and the accumulation of sentences applied for each manslaughter charge were excessive.

"The result was an aggregate sentence that was manifestly excessive and inappropriately 'crushing' in the circumstances of the case."

On the day of the crash, Davidson had been at home drinking and consuming cocaine and MDMA.

In the evening, he had driven to a liquor store and a petrol station, at one point travelling more than double the speed limit.

When he approached a bend, he lost control of his ute and hit the kerb at 111kph, striking the children.

Justice Adams said while the offending was a "very serious example of criminally negligent conduct", it was "important to consider that it was the one criminal act which caused so much harm".

"The sentencing judge was faced with a difficult task in sentencing the applicant for the highest number of manslaughter charges arising from the one act of criminal negligence in NSW to date.

"Despite this … I am satisfied that the aggregate sentence imposed is 'unreasonable or plainly unjust'. I would uphold this ground."

Veronique's mother, Bridget Sakr, said she was devastated by the reduction in sentence, describing it as "unjust".

"I'm in shock," she said.

"But you can't let the sentence define the value of our children's lives.

"It is a gut punch, but more than that, it's about other families ... we don't want other people to go through what we went through."

Her husband Craig Mackenzie said he felt the law was out of date.

"I think the judiciary is out of touch, I think something needs to change," he said.

At the time of Davidson's sentence, Judge James Bennett described his driving as "menacing" and a "horrific example" of misconduct.

"Tragedy was inevitable," Judge Bennett said.

"The magnitude of the tragedy, though, unimaginable."

Davidson could be eligible for parole from January 2035.