"This isn't right. This man gets all the protection, and he's committed child assault."
The assault occurred in late 2012, when her son, then aged 11, was living with his father.
The boy was thrown to the ground, hitting his head on a concrete garage floor, dragged by his neck, thrown against walls, and had his face smashed into the edge of a table.
He was hospitalised, and suffered a head injury which meant he was able to attend school only part-time last year.
Bay of Plenty police said they wrote to the victim's mother and met with her to apologise for a lack of communication, and failing to return phone calls or inform her about the change in charge and summary of facts.
Detective Inspector Mark Loper said there was no prospect of conviction on the more serious charge due to a lack of evidence, and the enquiry team were focused on securing a conviction and acknowledgement of guilt from the offender.
"Police take all assaults on children extremely seriously and this was no exception. A very thorough investigation was led by an experienced Detective Sergeant," he said.
Police accepted that the victim's mother could have been kept better informed throughout the investigation, he said.
Two senior officers visited her to discuss the outcome and her concerns, and an offer has been extended to meet the victim at the family's convenience.
But the victim's mother said during her two-and-a-half hour meeting with the senior officers, a lack of evidence was never mentioned.
She said police told her the downgrade was to avoid her son having to give evidence in court.
"There was so much medical evidence of a head injury," she said. "He was in concussion clinic for a year, he was hospitalised, there was lots of evidence."
She said her son wanted to give evidence, and wanted his father's name made public.
"He wanted to tell the judge what happened to him."
The woman said her son was still angry, and no longer trusted the police.
"He's very angry at the way he's been treated," she said.
- additional reporting by the Bay of Plenty Times