I disagree "una" can mean "a" as in this case the word "terza" and "bambina" both end in "a" so therefore it is approptiate to use "una". So for example to say the phrase "a turtle" you wound translatr that as "una tartaruga" as the word for turtle is feminine as is the case will "bambina" in this sentence's context.
bambino/a is a baby; ragazzo/a is a boy/girl; un/una giovane is youth/young adult i.e., young man/woman. That said, 'ragazzi' (only plural in this usage) can be used as a casual term for a group of students regardless of age, with the connotation of 'kids', though without the age-specific implication. I know this is how my teacher in a school for foreigners in Bologna would greet us every day, "Ciao ragazzi!" -- and trust me we weren't all young boys and girls! :-) Magari!
Another interesting question: how would a native Italian speaker understand this question? Does "una terza bambina" mean "the third girl" (i.e. there were two girls before and maybe a few boys) or "the third baby, which turned out to be a girl" (there were exactly two other children of any gender before)?