He put his hands up her top and began touching and putting his mouth on her body.
"She twice performed oral sex on you,'' Judge Philips said.
"You told police you knew that she was 13, and that you made a mistake.''
The girl no longer lived with her mother, and her victim impact statement spoke of the damaging effect Walker's behaviour had had on her daughter's self-esteem.
Her daughter had an attitude of "no longer caring'', Judge Phillips said.
The victim said because of the comments Walker had made to her, she was afraid he would act the same way towards other young girls.
"She just sees that she was totally taken advantage of,'' Judge Phillips said.
In a separate incident on May 13, Walker was in the Dunedin CBD with a group of other boys.
One of the boys left his bag in a CBD car park and Walker took it.
When the boy confronted Walker, he punched the victim in the face with a closed fist, before hugging him and asking him not to call the police.
"You seem to be a bully, really,'' Judge Phillips told Walker.
However those actions "paled into insignificance'' compared to his behaviour with the girl.
He noted that the pre-sentence report said Walker was "unsophisticated and immature'', and suffered from ADHD, oppositional defiant disorder, and foetal alcohol syndrome.
He sentenced him to four months' community detention and two years' supervision on the charge of sexual connection with a young person.
He also received two months' community detention and two years' supervision on the assault charge.
His supervision sentence included counselling and a curfew from 7pm to 7am.
He was also ordered to pay the male victim $100 in reparation.
The trial, which is being heard by Judge Bernadette Farnan, continues today.
