Katie Rough was found near her home in York, northern England, in January having been attacked with a box cutter and smothered, a case that grabbed headlines across Britain.
The defendant, who was 15 at the time of the attack, had been given a life sentence with a minimum five-year term of detention at a previous hearing in Leeds after admitting manslaughter due to mental issues.
Judge Michael Soole today called the case "truly exceptional" as he issued his ruling on the teenager, who appeared via video link, clutching a teddy bear.
"The gravity of the offence of killing a small child speaks for itself," he told the court.
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"The level of danger to the public is high, he added. "In the circumstances of your continuing silence, the critical question is whether there is any reliable estimate as to how long that danger will continue."
Rough was found with severe cuts to her neck and chest on a playing field, but her death occurred after being smothered, the court had earlier heard.
Prosecutors said the defendant was suffering from delusional thoughts, including the belief that people "weren't human and were robots".
The most notorious case involved Jon Venables and Robert Thompson, who were convicted of killing two-year-old James Bulger in Liverpool in 1993 when they were 10 years old.
Yesterday, Venables was ordered to return to prison after being found with child abuse images for a second time.
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