The 15-year-old boy inflicted 68 injuries on the child in 90 minutes, a jury at Manchester Crown Court heard.
Demi Leigh Mahon was attacked by the boy on July 15 last year.
Prosecutor Howard Bentham QC said that doctors believed the child was punched repeatedly inthe face before she died.
Paramedics who went to her aid said that she showed signs of having suffered brain damage as a result of being attacked.
The boy, who cannot be named for legal reasons, denies murdering the toddler.
Her mother, Ann Marie McDonald, told the jury she left her daughter while she popped out to pick up her child benefit at around 2.30pm. She said about 90 minutes later when she returned the boy was standing outside the house in Eccles, Salford, and told her: “I picked her up – I might have hurt her – and she fell in the park.”
McDonald, known as Sindy, broke down in tears when she told the jury how she rushed into the house to find her daughter lying in bed. “Her face was bruised. I tried to pick her up and her head flopped back. She couldn’t breathe properly.
“I screamed ‘Why haven’t you rung an ambulance?’ He said ‘I never done nothing, I’ve not done nothing’.”
The boy had looked after the child before, the jury of six men and six women heard.
Ann Dodsworth toldjurors she saw the defendant walking down the road holding Demi’s hand on July 15 and the child looked “okay”.
Paramedic Steven Blears said, in evidence read to the court, that when he arrived at the house Demi was lying on the floor and breathing infrequently.
He said Demi’s arms moved involuntarily towards her chest signifying brain damage had occurred. Blears said the defendant sat cross-legged in the doorway and told him that Demi had fallen in the park, but refused to answer his questions.
Blears said: “He said ‘I didn’t hit her, I didn’t batter her’.”
The female [Demi’s mother] was in a rage and said ‘I should never have left her with you’.”
He said the defendant then admitted he might have grabbed her “too hard” at the back of the neck while picking her up.
Blears said Demi had swelling to the back of the head, a bruised forehead, a ring of blood round her lips and a graze on her neck. He also said Demi’s injuries were inconsistent with the defendant’s description of how she sustained them.
PC Louise Warhurst said, in evidence read out, that shortly after arriving at the house McDonald shouted at the defendant “you hit her” and lunged at him.
PC Warhurst said the defendant told her Demi was bouncing up and down on the bed and fell off twice, first hitting her head on a glass ashtray and then on the bed’s headboard, after which she appeared to go to sleep.
Bentham said the defendant inflicted 68 injuries and a bite mark on the child.
Bentham said the defence will argue their client suffered an abnormality of mind which impaired his responsibility.
The trial, which isexpected to last a week, resumes today.