She told the Las Vegas Review-Journal she opposed the deal and had no word on it when it was announced in August.
“They knew when they stepped on my child's head that he was going to die. They are adults and they have to take responsibility,” she said. “They made an adult choice.”
Killers Trevion Randolph, Dontral Bieber and Gianni Robinson (all now 17) and Damian Hernandez, who turned 18 after the attack, were charged in January.
Nevada allows a 13-year-old to be charged with murder, but anyone convicted in the juvenile system in Clark County, which includes Las Vegas, can be held in juvenile detention until they turn 21.
If they complete a rehabilitation program, their parole period may end early.
District Attorney Steve Wolfson defended the plea agreement, saying it balanced the overwhelming facts of the case against the legal challenges that could arise if it went to trial.
Mr Wolfson added that a youth detention system would provide killers with better resources for reintegration into society.
