From my experience, "kiddo" is associated with sports, and is there (unfortunately) somewhat masculine. I've seen it usually said from men to boys, usually in closer relationships where the man supports the boy. So it could be said from father to son, or from a children's soccer team coach to a player. In movies, you might hear something like, "nice job, kiddo!" So often it's used approvingly.
However, it can still be used for girls in a similar circumstance.