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.
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.
In a statement, the boy’s family said Bravery’s crime against their son was “unforgivable”.
“Our son lives in fear of meeting other villains in his life,” they said.
The hearing continues.