"The girl is in the front and the boy is in the back."
Translation:A lány elöl van, a fiú meg hátul.
Yes, "van" is necessary here as the question is about location and not about attributes (predicatives).
Where is the girl: A lány hátul van. How is the girl (like): A lány szép
This is grammatically perfect, but Duo seems to prefer parsimony, and in this case you can omit the 2nd 'van'. Actually the sentence is more natural without the 2nd 'van'.
Why does meg goes after a fiu? It sounds to me as "the girl is in the front, the boy and is in the back
It's not wrong per se but you should use "meg" if you really want to express the same thing.
Is there a reason why meg is preferable, apart from the fact that the two of them are in opposite directions?
"meg" has nothing to do with directions, it's just the way to express this in Hungarian.