Mugen Shinshi: Bouken Katsugeki-hen

When a strange group of men attempt to kidnap Atsuko Fukune, a dancer at a club, she seeks the help of Mamiya Mugen, a famous detective - who also happens to be a child. When it turns out that other girls have gone missing, Mamiya, with the help of his butler Alucard, springs into action to solve the mystery.

(Source: Anime News Network)