"She is a child."
Translation:Cô ấy là một đứa trẻ.
Beta is beta. :P They are just using cô as a generic term for "she" at this point. Cô means aunt, specifically your mother's older sisters but it can also be used for any female that isn't a child.
It should technically be "em ấy là một đứa trẻ" as "em" meanings younger sibling.