In its report, the Child and Adolescent Mental Health Service spoke of the significant emotional harm the victim had suffered and that he would need intensive therapy.
The service said the child was "one of the most emotionally damaged children we have encountered", Judge Cooper said.
The judge said he did not accept submissions by Naera's lawyer Tim Barclay that the harm suffered by the child was caused by him abruptly being removed from Naera's home.
Judge Cooper said he was satisfied Naera's actions had contributed significantly to the impact on the child.
The age of the child, his vulnerability and that he was defenceless were matters which needed to be taken into account. There was also a significant breach of trust.
Naera had also attempted to conceal the assault by telling the boy not to say anything and to tell people he had fallen over at school, Judge Cooper said.
Naera continued to deny she assaulted the child and showed no remorse, Judge Cooper said.
The assault involved one single blow with the broom or length of wood.
Mr Barclay said Naera had not acted out of malice or deliberate cruelty and she admitted in her evidence that she was strict. Naera missed the child considerably and hoped to have contact with him in due course.
Mr Barclay said a starting sentence of one to two years' jail would be appropriate and given that range he asked Judge Cooper to consider home detention.
Rotorua Crown prosecutor Chris Macklin said any contact between Naera and the child would be inappropriate.
Judge Cooper said home detention would not be an adequate response to what was a serious assault on a child.