A 10-year-old Southern California boy is in a juvenile detention center after denying in court Thursday a charge that he murdered another child, authorities said.The mother of the alleged victim, Ryan Carter, said on HLN's "Nancy Grace" show that the suspect was her 12-year-old son's "best friend." Lisa Carter, a registered nurse, said that her son died after stepping in front of the 10-year-old as he was trying to stab another child."The things we taught him cost him his life," Carter said of her son. "We taught him don't start a fight, don't run away from a fight (and) if you see a fight ... stop it."The San Diego County Sheriff's Department received a call shortly after 1 p.m. Monday about the stabbing in Lakeside, which is about 20 miles northeast of San Diego.Despite efforts by sheriff's deputies, paramedics and hospital staff to save him, the boy was pronounced dead around 2:30 p.m. Monday, the sheriff's department said in a statement.The San Diego County District Attorney's Office filed murder and felony charges Wednesday against the 10-year-old suspect, said Steve Walker, a spokesman for that office.An attorney for the young suspect "denied the charges" during his arraignment Thursday morning, according to Walker. The boy -- whose name has not been made public, in accordance with the district attorney's policies regarding juvenile suspects -- is now being held in a juvenile detention center in the county, he added.Lisa Carter said that the boy who stabbed her son was actually targeting another child."Ryan took the knife for that boy," she said, describing her son as intelligent, caring and "everything to me."The victim's father, Glen Carter, called his son a "good ... boy with Christian values."Since he is a juvenile, the boy accused of killing Ryan Carter cannot be found guilty but can be convicted on a "true finding of fact," explained Walker.In convicted, he would have to be released by the time he turns 25 years old, if not earlier, the district attorney's spokesman added.The boy's next court date -- for a "readiness" hearing to determine if all sides are prepared to move forward -- is scheduled for February 23.