Actually the expression "we are no adults" is completely correct in my ears. Just transfer it to other expressions "We are no musicians /we are no vegetarians." Should be accepted in my honest opinion.
Ah, in that case, that sentence would be translated as «(Nós) Não somos nenhuns adultos.».
Sorry for the repetition, but just to clarify, do you mean like... "We are no adults"? I don't think that's grammatically correct, I've heard it said before, but then its a slangish way of saying it. :s
Technically in English, we sometimes use the phrase "We are no..." but it's rare. An exception would be "We're no angels" or "I'm no angel" which means "we aren't perfect" or "I'm not perfect".
Another thing we use it for is for profession. "I'm no scientist, but I think the Earth is round". or "I'm no mechanic but the car has a problem"
You were not that far off, but in this instance we would use "not" because it negates the noun and tells us whether or not someone is or isn't a child.
To me, when using no, sounds like "we are unadults" or "we are unangels" and etc.
"We are no adults if we can not calmly discuss this". "I am no expert, but I think...." The "we are no" formulation is less usual but perfectly correct.
It carries a meaning similar to, for example, "I am none of the types of existing experts", or "we are not any kind of people who can be classified as adults". Hope this makes sense :).