Bianca Jagger turned into a New York superhero last weekend. The activist and former wife of Rolling Stones lead singer Mick Jagger was walking across 57th Street at Park Avenue with a friend, we’re told, when she saved a small child. “There was a woman pushing a stroller who realized that a bag had fallen while crossing the street,” explained a spy, “and she ran back into the road to retrieve it . . . That’s when Bianca noticed that the young boy, age 2 or 3, was getting out of the stroller. Seconds before he ran into the road [after his mother], Bianca swooped in and grabbed him!” We’re told the grateful mom “kept on saying ‘Thank you! Thank you!’ ” Jagger couldn’t be reached for comment.