(1) "to appeal a little girl" is not good English, and I don't know what you mean by it. (2) 'kiddo' is a very informal British slang term used to address a young person and is not usually used in a complementary way; personally I have only rarely heard it used since probably the 1980s.

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.

"Kiddo" is a very unfornal way to refer to young people