Cops are investigating the death of a helpless 5-year-old Brooklyn girl who had spina bifida yet was left alone in a bathtub, sources said.

Little Destiny Delgado was in the custody of a Bed-Stuy couple that had been investigated by the city’s Administration for Children’s Services, police said.

Legal guardian Pedro Guzman swore he just left her “for a minute’’ Thursday to check on his choking dog, sources said.