A two-year-old boy and his 10-year-old sister are critically ill after they were knocked down in a hit and run collision, police said.
Detectives urged the driver of a silver Vauxhall Astra to come forward after the crash on Brudenell Road, Hyde Park, Leeds, at around 5.20pm on Saturday.
It happened as the children were crossing the road with another youngster near the Makkah Masjid Mosque, West Yorkshire Police said.
The boy and girl were taken to Leeds General Infirmary with what police said were serious injuries, and they remain in a critical condition following surgery.
Detective Superintendent Paul Taylor of the homicide and major enquiry team, who is leading the investigation, said: "We are in the early stages of investigating this incident but have already secured CCTV which detectives are studying, and spoken to witnesses.
"Wide-ranging inquiries by detectives and specialist officers are under way to trace the driver of this car and I would urge that driver to come forward and contact us now before they receive a knock on the door."
He added: "The family of these youngsters are understandably extremely distressed and for their sake we are keen to trace this driver as soon as possible.
"If it is a relative or a friend of yours and they have told you, then please pick up the phone and call us as a matter of urgency."
Information can be given to the homicide and major enquiry team on 101 or anonymously to CrimeStoppers on 0800 555 111.