The boy's mother Lisa Smith and her boyfriend Keiff King are charged with first degree murder and related charges.
Tahjir Smith died inside the couple's Willow Grove home last year.
Investigators said the couple repeatedly hit the boy after he spilled cereal.
According to Smith's confession, she and King were disciplining the boy for spilling his cereal and made him get into a push-up position. She said they then "laid the boy on the bed with his pants and underwear removed, grabbed a blue Levi's sandal and repeatedly struck him."
She said King "punched Tahjir in the back of the head," before putting him in a scalding shower and burning him.
According to court documents, Tahjir became unresponsive in the home and went in and out of consciousness. Police said that's when his mother called police and then began to walk down the street with the child.
According to an affidavit, Smith, who was six months pregnant at the time, was found wandering around outside her house with the 4-year-old lying nearby.
Paramedics arrived to find the boy limp and unresponsive. The boy was taken to Abington Memorial Hospital where he was pronounced dead.
A jury for the trial was seated Monday.