Watson handed himself into the Flaxmere police station two days after the accident.
Watson's defence counsel Matthew Phelps told the court his client had been carrying a ``heavy burden'' since the accident.
``This is a tragic situation for the victim obviously first and foremost and the victim's family and that's a sentiment Mr Watson expresses through me.''
He said Watson's conscience had told him to stop but he ``regrettably did not have the fortitude to stand up to others in the car telling him he should not go back''.
When spoken to by police, Watson said he had seen the child but had been unable to stop, then was afraid to return to the scene because of all the people in the street.
Immediately following the accident, he drove around the corner and parked the car at a friend's property.
Judge Jonathan Down said the police had not been able to determine whether the child or Watson was at fault in the circumstances of the accident, but said Watson's "criminality'' lay in his decision to leave the scene.
``It's not been possible to determine who was at fault but the young man suffered significant injuries that day.''
He said the suffering of Kayzah-Shae Wairama's parents had ``no doubt'' been made worse by Watson's decision not to stop.
``This is a situation we see in New Zealand from time to time that causes the New Zealand public a great deal of concern and anxiety.''
Judge Down said a starting point of nine months reflected the seriousness of the injuries the boy sustained.
He said a sentence of home detention would not be appropriate and it required an "immediate sentence of imprisonment''.
