"I am not so childish."
Translation:Eu não sou tão infantil.
No, at least not in this sentence. It would be better "eu não sou tão criança" or 'infantil'. Criancice is related to a situation, not to a person. 'Que criancice' - "what a childish"/"what childishness". "'Criancice' isnt an adjective, but 'criança' and 'infantil' may work as adjectives....
I thought that too. If you are childish, that behaviour is not necessarily permanent so I would have thought 'estou' to be OK.
I wondering about this, too. I chose sou, because if I'm making this statement, I want the person hearing it to think of it as the immutable truth.
...but it ain't necessarily so.