"Where is the bathroom?"
Translation:Onde é o banheiro?
I don't know if there's any rule that explains it, but when it comes to places or immovable things people usually say "onde é" or "onde fica", while for things that move or can be moved around like people or objects they use "onde está". That's because the meaning of the verb "ser" is usually permanent, while the meaning of the verb "estar" is usually temporary.
So unless it's a public bathroom with wheels and someone is moving it around, it'll just sound weird.