The teenage detective (Emma Roberts) promises her lawyer dad (Tate Donovan) that she’ll refrain from sleuthing while they’re living in Hollywood. But she can’t resist investigating the mysterious death, 25 years ago, of the movie star who lived in the house where they’re staying. Some of the three generations who’ve grown up with “Carolyn Keene’s” heroine may bridle a bit at seeing her turned into a slightly obsessive-compulsive little know-it-all, but probably not all that much; this is still recognizably the Nancy of the books. The movie is good fun for kids of all ages, with a sprightly pace and a clever original script by Tiffany Paulsen and director Andrew Fleming (plus a few amusing star cameos). Young Roberts has some of the girlish likeability of her Aunt Julia in her own early pictures.