When I first read the manga and saw Crona, I thought s/he was a girl. Then I though s/he was a boy. So I'm gonna stick with a boy, cause it's cuter that way.
And just so you know:The author has stated that he neither knows nor cares what Crona's true gender is. Any cases of "guy" or "daughter" in fan translations are so far just the translator's guesses.
The official English dub calls Crona a boy, but only because the writers (who are just as clueless) didn't know what else to say to avoid making the dub sound awkward.