Formal:
* מאַידל (Maydl): This is the most common and neutral word for "girl" in Yiddish. You can use it for a girl of any age, but it's slightly more formal than the other options.
* קליינע מאַידל (Kleyne Maydl): This literally means "small girl" and is a more formal way of saying "little girl."
Informal:
* מיידעלע (Maydele): This is a more affectionate and informal way of saying "little girl." It's often used for a young girl, but can also be used as a term of endearment for a woman.
* קלייניק (Kleynik): This is a very informal and playful way of saying "little girl." It's often used for a very young girl, and is sometimes considered to be a bit condescending.
The best option for you will depend on the specific situation.