A 9-year-old boy stabbed to death by his teenage babysitter was found dead on his bedroom floor by his mother, a court has heard.

The babysitter, who was 15 at the time, yesterday pleaded guilty to one charge of murdering the child on October 30 last year in the small Southland town of Otautau, 40km north-west of Invercargill.

The youth offender will be sentenced on May 7 in the High Court at Invercargill.

Interim name suppression orders covering both the defendant and victim remain in place until sentencing.

The summary of facts, released to the Herald today, says the youth had been paid to babysit the child on at least 10 previous occasions.

The child's mother had gone out that Wednesday evening about 6pm for a regular social outing.

She texted the babysitter at 10.10pm to check if her son was asleep.

However, the babysitter did not respond.

At about 7.40pm, he'd been seen walking along a street and at 8.20pm sent a Facebook Messenger note to a friend which said "help".

Ten minutes later, he sent a Snapchat message to three friends which said, "What would you do if I killed someone?"

At 8.35pm, he sent a Facebook message to another friend which said, "What would you think of me if I killed someone" and then a second message saying, "straight forward answer".

Sometime before 9pm, he was seen walking south along Riverton Otautau Rd and at 10.30pm, texted his mother to say, "I'm sorry come get me."

When the victim's mother returned home at about 10.30pm, she went to check on her son.

She looked in his bedroom but did not see him in bed.

"She went to turn away from the bedroom, but then stopped and pushed his bedroom door open," the summary says.

The mother then saw the boy lying on his back on the floor at the foot of the bed. He had a large knife in his stomach and he appeared to be dead.

She ran from the house screaming.

Emergency services rushed to the scene but the boy had been dead for some time.

An autopsy indicated the child had been killed within 90 minutes of being in the care of the babysitter.

The child had suffered three large knife wounds to his torso and chest area. He also had marks around his neck that were consistent with asphyxia as well as a moderate head injury.

The babysitter arrived home just before 11.30pm and was arrested by police an hour later.