A teenager “smiled and shrugged” after throwing a six-year-old from a 10th floor balcony at the Tate Modern, a court heard.
Jonty Bravery, 18, searched for the most vulnerable child at the London art gallery before “scooping up” the boy, the Old Bailey was told.
Prosecutors said the defendant, who admits attempted murder, had planned an attack well in advance.
A judge will pass sentence on Friday after hearing psychiatric reports.
In her opening remarks at the two-day hearing, prosecutor Deanna Heer said Bravery arrived at the Tate Modern on 4 August and made his way to the viewing balcony.
The court heard that CCTV footage showed him following young children and looking over railings.
Further video showed the victim – who had been visiting London from France – skipping ahead of his family along the platform towards Bravery.
Ms Heer said: “The defendant scooped him up and, without any hesitation, carried him straight to the railings and threw him over.”
The boy “fell head-first towards the ground”, landing on a fifth-floor balcony below, she said.
“CCTV also shows the defendant backing away from the railings, he can be seen to be smiling,” Ms Heer said.
“He has his arms raised and appears to shrug and laugh. CCTV also captured the parents in disbelief and rising panic.
“They thought there must have been a net but saw their son’s distorted body.”
The boy, who still requires round-the clock care, was lucky to survive, said Ms Heer, who described the attack as “a whisper away from a murder”.
She said the defendant admitted planning to commit an offence and had “narrowed it down to three possibilities”.
These were “strangling a woman or a child, drowning a child or throwing someone off a tall building”.
Ms Heer said Bravery’s search history included results for methods of killing.
The prosecutor said medics appeared to agree that the defendant suffers from Autism Spectrum Disorder (ASD) and a personality disorder, both of which are relevant to understand his behaviour.
Ms Heer said Bravery blamed social services when challenged by distraught witnesses moments after the incident.
The court heard Bravery has no previous convictions but had a history of assaulting his care workers – including one at a Burger King in Brighton in April 2019.
BBC News correspondent Helena Wilkinson
Jonty Bravery appeared via videolink from Broadmoor hospital to hear the prosecution outline the horrifying events for which he was responsible for last August.
Sporting a beard and wearing a white T-shirt with blue shorts, he sat in a chair for most of the hearing.
At times Bravery appeared focused. At others times he seemed distracted. At one stage he could be seen with his T-shirt over his head.
As he listened to details of what he did at the Tate Modern last year, Bravery moved from his chair and crouched on the floor to face the wall.
The hearing took place under strict social distancing measures with everyone in court sitting 2m apart. The dock, where defendants normally sit, was used as extra space for media.
The boy’s family said Bravery’s crime against their son was “unspeakable”.
In a statement taken six months after the attack, his parents said they had maintained a near-constant bedside vigil at the Royal London Hospital.
This continued, they said, after he was transferred to a French hospital in September, with the family only returning home on rare occasions to collect belongings.
“We were forced to sleep in very rough conditions in the ‘family room’ of one of the hospitals, on a mattress on the floor, in a draught, before finding an AirBnB nearby offering better sleeping conditions and a place where to finally put down our things,” they said.
“In all, we’ve stayed in eight different accommodations so far and are about to move once more.
“We have been so scared of losing him that now it is impossible for us to spend more than a few hours away from him,” they said.
They said their son was unable to trust people, and said he “would like to slap” Bravery for what he did.
“Our son lives in fear of meeting other villains in his life,” they said.
Dr Joanna Dow, a consultant forensic psychiatrist who works at Broadmoor Hospital where Bravery is being held, said she believed the Bravery had a mixed personality disorder and struggled to manage his emotions.
She recommended he be detained in hospital, rather than handed a prison sentence, so he could get treatment for anger management and help to learn social communication and interaction skills.
Bravery’s defence barrister Pippa McAtasney QC said at the time of the attack the teenager had been in the care of Hammersmith and Fulham Council.
She referenced a recording obtained by the BBC which revealed Bravery told carers about his plans to kill a year earlier.
Ms McAtasney said letters from his parents revealed to the court that Bravery was diagnosed with autism and “is a loved child”.
“Both parents strived to improve the quality of his life and secured and managed environment for their son,” she said.
“They had no inclination their son would commit such a shocking crime. Those responsible for his care never communicated the contents of the shocking, prophetic recording that was revealed through the media.”
The hearing continues.