"La niña está cansada."
Translation:The girl is tired.
I guess being tired is considered a "condition" and that is usually covered by the verb "estar"/"está".
Yes, she will not be tired after a sleep, say - it's how she is now. But interestingly, if "es" is used it is a characteristic not a condition so she is the sort f person who is always tired (I imagine in a Victorian novel) or, even better, I have seen it translated as "tiresome"! Would welcome a native speaker clarifying meaning of "ser cansada"