Former Nickelodeon child star Kianna Underwood has been killed in a hit & run in New York.

Law enforcement sources tell TMZ the 33-year-old -- who appeared on the 2005 series "All That" -- suffered severe trauma to her head and torso ... when a gray vehicle, driven by an unknown suspect, was traveling west on Pitkin Avenue and struck her as she crossed the street at Pitkin and Mother Gaston Blvd.