Police are searching for a person believed to have thrown bleach at a toddler from a moving car in Sydney's north-west this afternoon.

About 4.45 pm, a mother was walking on Conrad Road in Kellyville ridge, pushing her six-month-old daughter in a pram and holding her four-year-old daughter's hand.

A passing small red car threw a bottle at the group.

A liquid, understood to be bleach, splashed the four-year-old girl on the face.

She was taken to the Children's Hospital at Westmead for treatment for minor injuries and skin irritation.

The girl is in a stable condition.