Formal/Neutral:
* Merch fach: This is the most literal translation, meaning "girl small."
* Merch ifanc: This means "young girl," and is often used for a slightly older girl than "merch fach."
Informal/Affectionate:
* Bachgen bach: This means "little girl," but it's more playful and affectionate than "merch fach."
Other Options:
* Plentyn bach: This means "little child" and could be used to refer to a little girl.
* Merched bach: This means "little girls" and can be used when referring to multiple girls.
Important Note: Welsh is a very nuanced language, and the best way to translate "little girl" will depend on the specific situation and the relationship between the speaker and the listener.