The woman was behind the wheel of a white Volvo sedan heading downhill on Ninth Street towards Fifth Avenue at 12:41 pm, when she allegedly ran a red light and slammed into the women and children, according to witnesses.
Pix11 reported that a group of people got off a bus onto the sidewalk when a driver who appeared to be speeding up to make it through a light slammed into them.
NBC4 chopper video from the scene shows a white vehicle which appears to have crashed head-on into a parked sedan.
The accident happened at 9th Street and Fifth Avenue, reported ABC7.
Two children - a 1-year-old boy and 4-year-old girl - were killed.
The FDNY says two other victims were transported to a hospital. Their pregnant mom is in critical condition.
The driver was taken into custody. "Unimaginably terrible", he said on Twitter.
New York City Mayor Bill de Blasio said that five people were taken to a local hospital, including the children who died. Ruthie Anne Miles is expected to survive, though reports about her unborn child's condition were unclear.
A stroller belonging to one of the young victims lay mangled in the roadway alongside a sneaker and an assortment of children's books and accessories.
Eric Adams, the Brooklyn borough president, said via Twitter that he's monitoring the situation.
Two other adults were injured in the incident at lunchtime in Park Slope.
Police officers told CBS that they're checking to see if the driver was under the influence while operating the vehicle.