A man who told his grandson he "needed a good kick in the backside" ignored the boy's warnings about calling the police and did it anyway, earning himself community work in Whanganui District Court.
The man's daughter placed her 13-year-old in his care because of behaviour difficulties, said defence lawyer Stephen Ross on Tuesday.
Police prosecutor Sergeant Drew Morrison said the man received a call from the boy's school on November 17, 2013 informing him his grandson had been skipping classes.
He "interrogated" his grandson and, upon getting no answers, turned the boy around and "kicked him twice in the backside", said Mr Morrison.
Mr Ross said the grandson had continually misbehaved at school.