Translation:We have two children, a boy and a girl.
we have two children, one boy and a girl
although it is not the same I think that it should have been marked as correct.
o fata - could be interpreted as "a girl" or "one girl"
do you have to have a match between - one boy - one girl - / a boy - a girl. Is that why it was marked as incorrect.
there is no difference in Romanian between ”one” and ”a”, the difference is only in English
As a native English speaker, I would say "one boy and a girl". To me it sounds more natural.
There may be a preference (though I'm sure there is some actual English rules that exist) and depend on which part of the U.S. you grew up in, but as a Native English speaker myself it sounds more natural if you use spelled words that match (one boy/one girl or two boys/two girls).
With the exception if you really have only one of something (two boys/a girl) would sound fine, but would still probably be more proper/formal to say (two boys/one girl).