HAGERSTOWN, Md. -- The uncle of a 9-year-old boy fatally beaten over a missing piece of birthday cake pleaded guilty to first-degree child abuse Tuesday, acknowledging he handcuffed the child to a chair and allowed the mother's controlling boyfriend to pummel him senseless.

When the beating became so brutal that Jacob Barajas couldn't bear to watch, he closed his eyes, Assistant State's Attorney Sarah Mollett-Gaumer told the court. Barajas called 911 but the boy's mother sent the ambulance away, delaying treatment for hours.

Barajas, 24, faces up to 15 years in prison. The maximum penalty is 25 years, but prosecutors agreed to a lesser sentence and dropped six other charges, including second-degree murder, in the plea deal.

Barajas may be called to testify against the boy's mother, Oriana Garcia, 27, at her trial in February, Deputy State's Attorney Joseph Michael said.

Garcia's boyfriend, Robert Wilson, 31, is serving a 30-year sentence after pleading guilty in March to second-degree murder in the July 2015 death of Jack Garcia.

Prosecutors and defence attorney Stephen Sachs revealed new details Tuesday about the months of abuse Jack endured after his mother moved them and her brother from Goleta, California, to Hagerstown, about 113 kilometres west of Baltimore, to live with a man she had met online.

Garcia didn't enrol Jack in school in Maryland, partly because his legal custodian was Garcia's mother, who lived in another state, Sachs said. He said Jack's father had never seen the boy.

Barajas, a 10th-grade dropout, had lived with his sister and nephew for years, caring for Jack while Garcia worked in retail. But when they moved in with Wilson, a restaurant cook with a history of violence, all three fell subject to his strict household rules.

Even at his sentencing, Wilson complained about people in the apartment "not doing what they were supposed to."

Wilson enforced discipline by controlling food servings, Sachs said. Jack, described in an autopsy report as thin, began taking and hiding food, according to lawyers for both sides.

Stealing food brought physical punishment. Garcia told police she allowed her son to be handcuffed, sometimes for hours, and that Wilson had slapped the boy, hit him in the leg with a bamboo sword and sparred with him in the weeks before he died.

On the day Jack was fatally beaten, "I think he was just hungry and Wilson went off his rocker on that," Sachs said.

The beating followed the disappearance of a piece of cake at a birthday party for Wilson's visiting 2-year-old daughter, according to police reports. Another child told investigators she heard Wilson yelling, "Cough up the cake," as Jack cried, "No! Stop!"

Sachs said Barajas may have felt helpless to stop the beating.

"I think they were scared to death of him," he said.