Please take this with a grain of salt as I have no knowledge of
German language, at worst this is good chance for
German speakers to laugh at me ;)
If you want something literally "little fire," my first thought was "Flammelein," where die Flamme = flame and lein = affectionate diminutive suffix. I am not sure, but I would guess this nickname would have a poetic, maybe flowery or outdated connotation. I did a Google search and only found it in a poetic context:
https://lueersen.homedns.org/!gutenb/arnim/wundhorn/wh2050b.htm
You might also consider a nickname tied to
Helen, which has the possible meaning "torch."
Ilka,
Lena, and
Ella are diminutives of either
Helen or names etymologically linked to
Helen.