Formal:
* Ma petite princesse: This is the most literal translation and is suitable for a formal setting.
Informal:
* Mon petit chou: This is a term of endearment for a little girl, meaning "my little cabbage."
* Mon petit ange: This means "my little angel" and is another common term of endearment.
* Ma puce: This is a more affectionate term, meaning "my little flea."
* Ma petite chérie: This means "my little darling" and is a very affectionate term.
Choosing the right phrase:
* "Ma petite princesse" is best if you want to be formal or if you're talking about a young girl in a more neutral context.
* The other phrases are more affectionate and suitable for close family or friends.
You could also use "Mon petit trésor" which means "my little treasure" for a very special little girl.
Remember that the best way to say "my little princess" in French is the phrase that best fits the specific situation and your relationship with the person you are talking about.