Choudhury added: “You headed straight towards her, pulled out the knife and as soon as you approached the unsuspecting Lilia, you plunged the knife into her heart.”
He said the CCTV from before the incident showed Lilia as a “carefree, happy girl” who “should have been safe”.
He added: “How could anyone imagine that within seconds she would be stabbed by a complete stranger and left to die?
“Yours was a shocking and horrific act of violence.
“Nothing I can do or say today can relieve the pain of losing Lilia.”
Skebas, who appeared via videolink from Rampton Hospital in Nottinghamshire, was told he would remain at the psychiatric facility.
However, the judge said if at any point Skebas was deemed well enough to be discharged he would be sent to prison to serve the remainder of his sentence.
During the trial, the jury heard Lilia had been playing in Fountain Lane.
Skebas, formerly of Thorold Street, Boston, ran towards Lilia and stabbed her once in the chest.
Her mother, Lina Savicke, told the court Lilia called her name and after she rushed outside the girl collapsed into her arms.
Despite the efforts of an off-duty police officer, doctors and paramedics, Lilia died within an hour of being attacked.
