Authorities were aware a father, whose neglected four-year-old was found locked in a filthy room and two days from death, had previously had a child removed from his care, an Adelaide court has heard.

The 28-year-old father and 24-year-old mother have pleaded guilty to an aggravated charge of endangering the life of their son, who weighed just 8.3kg when found naked and starving by police last October.

The mother told police she was "too busy having sex" to care for her son.

"I've messed up, I've done the wrong thing. I know I shouldn't have thought about my man before my son," she said.

"I didn't associate with my child like I should have, instead I was too busy having sex with (her partner)."

Photos tendered to the court show a bathroom in the Adelaide home where a four-year-old boy was found naked and starving. (Supplied)

The discovery of the boy by police was “completely fortuitous” after his mother reported her partner was assaulting her, the prosecution told the court.

The court heard the father had caused the mother to miscarry an earlier pregnancy when he kicked her in the stomach.

Doctors say the boy was just days from death when he was found. (Supplied)

Authorities had removed a child from him when he lived in Victoria but there was no communication between that agency and Families SA.

Judge Paul Cuthbertson wants to know why agencies didn't rescue him sooner.

"You'd only have to see the child once to work out there was something wrong. You wouldn't even have to be an expert," he said.

The District Court was told the parents had been on a binge of alcohol and cannabis and had fed the boy yoghurt, custard or jelly on plates they slid under the door.

Officers had to walk through garbage bags, rotting food, dirty nappies and sanitary pads to get to the boy, who was surrounded by used nappies, plastic plates and his own faecal matter.

The court heard the mother posted photos of her dinners on Facebook when her son was locked away, starving.

Photos from social media purport to show her cooking her first roast lamb and experimenting with melted chocolate desserts.

The court heard the mother posted photos of dinner on Facebook while her son was locked in a room starving. (Supplied)

This photo shows the kitchen in the home, strewn with energy drinks. (Supplied)

Doctors believe the boy’s dehydration level was at 10 percent and two days away from reaching a critical level of 15 percent.

Prosecutor Lucy Boord said the case is one of the most serious criminal neglect cases the court is likely to see and has called for a lengthy jail sentence.

A victim impact statement, compiled by authorities and the boy’s foster parents was read in court today.

The boy's mother cried uncontrollably as it was read.

It said normal family outings were obviously new to the boy, who would not come out of his room for the first six months after his removal.

The parents of the child found in the home have pleaded guilty to endangering their son's life. (Supplied)

He has had multiple decayed teeth removed, is still being toilet trained and cannot control his emotions, sometimes punching himself in the head or running into walls.

When he wakes up, he would lie in his bed waiting for someone to come into his room.

He cannot tolerate doors being closed, has limited language skills and hides food in his pockets.