Adams indecently assaulted the girl a second time when he was looking after her, Judge Harding said.
The judge said the impact on the victim was huge and she described the offending as changing her life, stripping her of her childhood and teenage years. She was unable to sleep and suffered nightmares.
"This offending was a gross breach of trust against a particularly vulnerable victim," Judge Harding said.
Outside court, his daughter said they had travelled from Northland to see Adams' appear "but we aren't here for him".
"I wanted to come to hear first-hand what he's done and so he could see my face and feel regret. I mouthed the words 'I hate you' and he just looked at me 'Oh, c'mon'. He gave us the look like he didn't do this."
Adams' daughter, who would not be identified, said there needed to be more reports of this kind of offending to encourage other victims to come forward.
Through tears, Adams' sister, who also would not be identified, said the offending had destroyed the family.
"I wish he got 10 years with what he's done to that little girl," she said. "He's destroyed us. He's my only brother and all I've got and I don't want him anywhere near me or my grandchildren. I'll get a restraining order against him for the day he is finally released."
Crown prosecutor Heidi Wrigley argued Adams' premeditation and the victim's young age were factors to be considered in sentencing.
Defence counsel Bill Nabney submitted there was no premeditation and his client simply "took advantage of the situation as it presented itself".
Judge Harding disputed this and ordered 20 months' imprisonment for each offence, to be served concurrently.