A former Adelaide childcare worker has pleaded guilty to having sex with a 13-year-old boy after grooming him through Facebook.

The District Court heard Cameron Michael Stinear was 19 at the time of the offence in September 2014 and was on bail for child pornography charges.

Stinear, now 22, is on trial and has now changed his plea to guilty for unlawful sexual intercourse with a person under the age of 14.

His defence lawyer, Martin Anders, told the trial judge that his client had "shifted his instructions".

The court was told Stinear engaged in "deliberate, calculated, grooming and normalising behaviour" through sexual and graphic Facebook chats with the boy.

The 13-year-old later met up with Stinear and performed a sex act on him in his car.

Prosecutor Chris Edge said both parties appeared quite keen to meet up, but the boy's mother found the pair's messages to each other and contacted police.

Stinear has already admitted to a string of other charges, including communicating with the intention of making a child amenable to sexual activity and causing a child to expose a body part.

He also faces two counts of possessing child pornography.

Stinear will return to court in May for further sentencing submissions.