"She is not a girl."
Translation:Sie ist kein Mädchen.
Yes. Whenever you have an indefinite article or no article, you use "kein." http://german.about.com/od/grammar/a/German-Negation.htm
What Gcoid says. Mädchen is a neuter word, not female. All words ending in -chen are neuter; -chen is a diminutive although that is lost in Mädchen. "Männchen" is a little man and "Häuschen" is a little house.
As you may have noticed, an Umlaut is added to signify the change in sound. "Baum" > "Bäumchen". As with "Mädchen", the word doesn't change for plural, but the article does. "Das Bäumchen" > "die Bäumchen" (the little tree > the little trees). The same applies to "kein". "Sie sind kein Mädchen" (you are not a girl) > "Sie sind keine Mädchen" (they are not girls).