Investigators say the victims' car slowed down to avoid a raised metal sewer cap when it was rear-ended Saturday morning by a private sanitation truck in the Port Morris section.
The sedan was pushed into a metal pillar supporting the Bruckner Expressway in the 300 block of Bruckner Boulevard.
3-year-old Sophia Aguirre was killed.
Her mother and grandparents are in stable condition.
The garbage truck driver has not been charged.
Police said the child was properly fastened into the rear seat of the vehicle.
The carting truck is owned by Avid Waste Systems, of Hunts Point. in the Bronx.