Butler County Coroner Wayne Garlock has said the children's ages ranged from nine months to 17 years old.
He said the vehicles were likely to have hydroplaned on the wet roads and lost control.
Tropical Depression Claudette has caused flash flooding and tornadoes, destroying dozens of homes in Alabama.
Mr Garlock said a father and nine-month-old girl died in one of the cars.
The other eight victims were in a bus belonging to the Tallapoosa County Girls Ranch, a foster home for abandoned, neglected or abused girls. The youngest was four years old.
