You may use it for a girlfried (if she likes it) or for pet cats. Kätzchen comes from Katze (cat). Kätzchen is familiar form for Katze. Every expression with -chen ending has a cosy meaning in German, for example Spätzchen which comes from Spatz (sparrow).

I don't use it for girls. I guess other people would.

Hallo Erika, da Sie Deutsch lernen und den Film „The Shawshank Redemption" mögen, ist vielleicht dieser Text für Sie interessant: http://www.italki.com/entry/347571

Kätzchen = Kitty-chan And, yes, it's for girls.