"La niña es", no "La niña está". You are defining the girld, how is she

La niña es tan pequeña que no puede peinarse sola

La niña es muy pequeña, no puede peinarse sola. / Use the correct form of the verb and avoid the conjunctions like "que".

While the sentence sound better with "es" in this case it is acceptable to use "está" as well but only for informal language purposes. "Ser" is used usually for someone's permanent characteristics while "Estar" is used for transitive states/characteristics. For example, we say "Ella está enferma." Since usually sickness is a transitive state "estar" is used. Your example is kind of a gray area since children usually grow up but still someone's size is usually regarded as a permanent characteristic. So is it "ser" or "estar"? The short answer is ser since it just sounds better, in informal speech you can use any an no one will notice any oddity.