"What compelled you to do this? It's unfathomable," added Avitto, who had his son's initials freshly shaved into his hair.
Prince's mother, Aricka McClinton, told the Daily News that her son came to her in a dream and said: "Hold on, Mommy, they're going to get him."
"It feels like my baby can rest in peace," she added.
St. Hubert was picked up just after 8 p.m. Wednesday in a residential neighborhood of Queens, police said. It was unclear if he had an attorney.
Forensic evidence led to his identification, according to police, who didn't elaborate and provided no possible motive for the attack.
St. Hubert has a history of violent assault, with nine arrests on his criminal record, including two for assaulting police and correction officers, and he was released on May 23 in a domestic assault case, authorities said. He was sentenced to time behind bars after convictions for attempted murder and assault, serving three years in city jails and two years in an upstate prison, state correction records show.
Before the Sunday evening assault, Prince and Mikayla had been playing outside the Boulevard Houses under adult supervision. Relatives and witnesses said they went inside alone, where they were cornered by a heavyset man in his 20s.
The attacker used a steak knife to stab each child multiple times, police said. He fled on foot, leaving the knife outside the building, they said. Prince was found in the elevator, Mikayla just outside the building. Chicago Bulls power forward Taj Gibson says Prince was his cousin.
Police said they were investigating whether St. Hubert also may have fatally stabbed 18-year-old Tanaya Copeland on Friday four blocks from where Sunday's attack happened. A similar knife was recovered at the scene of Copeland's slaying.
The knife attack on the children has cast a spotlight on the issue of surveillance cameras because none was in place to capture any images of the attacker despite funds set aside by the New York City Housing Authority to pay for them.