Ugh I hated this book when I had to read it. It was good, but school has a tendency to make things terrible that would be good in any other circumstance.
The answer:
One way Linda Sue Park has created different points of view for Nya and Salva is that Nya and Salva are both so much alike and yet so different. Salva is with a large group of other 'lost boys' and has to walk for so long with barely any rest is escaping from the treacherous effects of war; while Nya is walking by herself everyday twice a day many miles to get water for her family barefooted.