Both are correct. In English, we generally don't apply masculinity/femininity. In Spanish, niña is the feminine version of child. So, saying "Ella es una niña." means "She is a child." with female being implied.
"él" requires an accent when it means "he", and no accent when it means "the". The accent is only there to differentiate the two meanings. "ella" has no other word that sounds like it, so it has no accent in this case.