The California Highway Patrol says the 10-year-old was driving with the six-year-old in the back when they overturned near Cedar St. and E. Excelsior Ave. just after 6 p.m.
It is unknown what caused the ATV to overturn. Authorities say neither of the girls were wearing helmets at the time of the accident.
Parents discovered the crash and called 911.
The 10-year-old was airlifted to Valley Children's Hospital in critical condition, and the six-year-old was taken by ambulance.