A two-year-old boy and a girl-of-four were seriously hurt after being hit by a car as they crossed a road with their mother, police said.
The pedestrians suffered "multiple serious injuries" on Burton Road in Upper Gornal, Dudley, just before 08.30 GMT on Tuesday.
They were taken to Birmingham Children Hospital for treatment.
A driver of a VW Polo is assisting officers with their inquiries, West Midlands Police said.
West Midlands Ambulance Service said they sent two ambulances, a paramedic officer and the Midlands Air Ambulance from Cosford to the scene.
A spokesman said: "Crews arrived to discover two patients, a young boy and a young girl who had been involved in collision with a car.
"The air ambulance crew, which included a doctor, split between the two ambulances to help continue treatment to both patients en-route to hospital."
Two children, aged 4 and 2, injured in collision while crossing Burton Road with their mother in Upper Gornal this morning just before 8.30am - the driver of a WV Polo is assisting officers with enquiries. If you saw what happened please contact Sgt Ade Brown at force traffic. pic.twitter.com/qLqkurDYw6— Dudley Police (@DudleyPolice) November 6, 2018
A West Midlands Police spokeswoman added: "Both have suffered serious injuries and the road has been closed as officers establish what happened."
Anyone who saw what happened should contact Sgt Adrian Brown from the force Traffic department on 101.