The accident happened at about 9:45 a.m. Monday in the eastbound lanes at the Pyrite exit. Three people were also critically hurt, and two others suffered minor to moderate injuries.
The Riverside County Coroner identified the children as 6-year-old Fernando Benitez of Los Angeles and 14-year-old Genesis Fuentes of Los Angeles.
A preliminary investigation showed the SUV rolled after an apparent tire blowout.
California Highway Patrol officials said only three people were property restrained in the vehicle.
The families were headed to Lake Perris where they planned to celebrate the 4th of July.
The Associated Press contributed to this report.