The incident happened just before 9 p.m. Saturday along Stoddard Wells Road near the Stoddard Wells vehicle area.
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Investigators say Jacob Martinez was riding an ATV (all-terrain vehicle) with his family friend, 11-year-old Christina Bird, when they were struck by a side-by-side UTV (utility task vehicle).
ATVs and UTVs are both off-road vehicles, but UTVs are generally larger with more powerful engines and can be used for commercial purposes in addition to recreation.
RELATED: UTV driver hits two children in ATV in Apple Valley
The driver of the UTV fled the scene and has not been identified.
Jacob was later pronounced dead at the scene. Christina was rushed to St. Mary's Hospital and was later flown to Loma Linda University Medical Center. Her family held a vigil at Loma Linda Tuesday night.
On Wednesday, they disclosed that Christina had died.
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The family is hoping the driver who left the scene with two small children lying on a dirt road comes forward voluntarily.
"I ask that if you were there with that person, or know of that person who was there ... saw what happened ... please be compassionate and come forward," said Martinez's sister, Cynthia Nunez. "Don't hide them."
The families of both children are asking anyone with information to call the California Highway Patrol in Victorville at 760-241-1186.
You can also submit an anonymous tip with Crime Stoppers at 800-222-8477.
GoFundMe accounts have been created for both the families of Martinez and Bird.