Judge Rea said the details about who switched on the video were disputed by Day, who said it was not him but the boy.
"It makes little difference - it was your home, your bedroom and it was you who should have shut it down," the judge said.
Once the video was on, Day committed an indecent act on the boy and himself.
Judge Rea said there were no physical injuries but there was a psychological effect "this type of offence would usually have on a 10-year-old boy".
Originally submissions had been filed to the court indicating a starting point of fewer than two years' imprisonment, which would make Day eligible for a sentence of home detention.
"I was prepared to consider that," the judge said.
However, after failing to find any other similar cases which had ended in a sentence of home detention, he said that type of offending must result in a prison sentence.
He said home detention denounced the conduct and sent a message to others that sexual offending against children did not happen. "Home detention simply does not reflect the seriousness of the offending."
The starting point reached was 20 months' imprisonment.
Day was given a discount for his early guilty plea and was handed a 15-month prison sentence.
He was also given his first warning under the three-strike rule.