Formal:
* Mon petit voleur (pronounced "moh peh-tee vo-lur") - This is the most literal translation and is appropriate for formal situations.
Informal:
* Mon petit voleur (pronounced "moh peh-tee vo-lur") - This is still a formal phrase but can be used informally.
* Mon petit cambrioleur (pronounced "moh peh-tee kam-bree-o-lur") - This is a more playful and affectionate way to say "my little thief," implying that the person is stealing your heart.
* Mon petit bandit (pronounced "moh peh-tee ban-dee") - Similar to "cambrioleur," this is a playful and affectionate term for a mischievous person.
Other options:
* Mon petit escroc (pronounced "moh peh-tee eh-skrok") - This is a less common term but can be used to refer to someone who is charming and cunning.
* Mon petit filou (pronounced "moh peh-tee fee-loo") - This is a more playful and affectionate term for someone who is a bit of a rogue.
The best way to choose the right phrase depends on the context and your relationship with the person you are talking about.