Formal:
* Mon petit canard français: This is the most literal and formal translation.
* Ma petite cane française: If you're specifically referring to a female duck, this is the correct translation.
Informal:
* Mon petit canard français: This is still a good option, but it can feel a bit more casual.
* Mon petit canard: You can omit "français" if it's clear from context that you're talking about a French duck.
* Ma petite cane: As above, you can also omit "française" if the context is clear.
Other options:
* Mon petit caneton français: This is a more endearing way to say "my little French duckling."
* Mon canard français chéri: This is a more affectionate way to say "my little French duck."
Ultimately, the best way to say "my little French duck" in French depends on the specific situation and your desired level of formality.