A Lowell mother and her 8-year-old son are being heralded as heroes after they recently came to the aid of a young girl in Chelmsford’s Freeman Lake. Jessica Levasseur was teaching her son, Devin, how to swim when the boy noticed a little girl floating nearby. He pointed her out to his mother, thinking she, too, was learning to float. Levasseur, a former nurse, reacted by pulling the unconscious 3-year-old girl from the water and bringing her to the shore, where she spit out water and regained consciousness. Levasseur then told friends to call 911; EMTs arrived at the scene to find the girl alert and sitting with her mother. Chelmsford Police have not identified her by name.