The family identified the boy as Jamarion Thomas, who was run over at about 7:10 p.m. in the 200 block of East 97th Street.
Holding onto a small black tennis shoe, Jamarion's grandmother, Lanora Satterwhite, told KTLA-TV it was all she had left.
"He's dead. He's dead," she said. "He was just walking the bike across the street."
Satterwhite rejected reports that Jamarion was holding on to the truck while on his bike, saying he was "too little to hold on."
"He was pushing it across the street," she said.
Los Angeles police Sgt. Timothy Colson, meanwhile, told reporters at the scene that Jamarion was riding next to the truck on his motorized bike when, for an unknown reason, he apparently slipped, was caught under a rear wheel and run over.
Neighborhood residents attacked the ice-cream truck and the driver, throwing bricks, sticks and possibly brandishing a knife. Jamarion's uncle told NBC LA that the boy's mother and nephews might have been involved in the attack.
Jamarion was transported to Harbor-UCLA Medical Center, where he was pronounced dead.
The ice-cream truck driver, who was not identified, suffered minor injuries and was also taken to the hospital, police said.
The driver, who was not under the influence of drugs or alcohol, has not been arrested in connection with the incident, police said.
Times staff writer Ryan Parker contribued to this report.