Superintendent Gabriel Cameron said none of their conditions were life-threatening, but their injuries could be life-changing.
A 31-year-old woman and her three-year-old daughter were seriously injured after corrosive acid was allegedly thrown at them in Clapham, south London.
At least nine other people, including an eight-year-old girl, were injured in the same attack, the Metropolitan Police said.
Both the woman and children were taken to hospital, as were three other people who were injured after they came to their aid, Scotland Yard said in a statement.
Detective Superintendent Alexander Castle said three police officers were also taken to hospital after arriving at the scene in Lambeth, but their injuries were minor.
Castle said tests were underway to determine what had caused the injuries, but “at this stage we believe it was a corrosive substance”. Police are still searching for the attacker seen fleeing the scene.
The police air service also took part in the operation, for which help was also requested from the population.
Marina Ahmad, a member of the London Assembly, said both the women and the injured girl were passengers in the car.
CCTV footage shows a man running around a car before getting into the driver’s seat. A woman and her child are sitting in front of the car.
The images show the car hitting the woman and then the man stopping the car, getting out and aggressively grabbing the child.
After the accident, the man attempted to flee in his car, but collided with another vehicle and fled on foot.
“While none of their conditions are life-threatening, the woman and child’s injuries could be life-changing,” Superintendent Gabriel Cameron said.
“It may be some time before hospital staff are able to say how serious the situation might be,” he added.
One of the witnesses told Sky News what he saw: “The man violently grabbed one of the two children and slammed them to the ground.”
“The lady then shouted: ‘My eyes! My eyes! Call the police, my eyes! Then I saw him run away. It was all so traumatising.”