Many thanks to the FREE Duolingo for the good work they made. A mistake is human, to give a lot of compliments as well(!) Don´t only complain here, but report the mistake as i did already. Remember: you are using a lot of work from voluntears FOR FREE!
Good advice, but what is the mistake here? I ask because I can't see Duolingo's translation; is it not "The girls write"?
No, the (wrong) translation is "the girls write a book", but they don´t write anything about a book in the question.
how can I say They ( girls and boys) write? Cause I see they have a diversification for Meninos and Meninas, so what happens when the group mixes?
Portuguese almost always uses the masculine form for mixed groups so it would be "Eles escrevem" even if the majority of the group were girls. Of course, there is no reason why you can't be more explicit and say "Os meninos e as meninas escrevem".