Translation:They are all girls.
Maybe someone asked you about a group of people they don't know very well. Then you reply with this sentence. They are all girls. But then you still need to use correct form of "they" obviously, and you can't just say "They all are" either. Like when someone asks you "Is this person a boy or a girl?" and you would have to say "She is a girl", even though "She" already implies the gender.
This is exactly what I thought too. Or a large enough group that it's not trivial to verify all the ages. Perhaps the girls are lined up for a ride and need children's seats, or someone is enquiring about the group eating off the children's menu at a restaurant. There are a hundred reasons to say this in this way.
女の子、男の子、are usually children. Teenagers can be called this way, but if a person is working, he or she is not 男の子 or 女の子even if he or she is a teen. If your boss calls you this way, you are considered as a novice or your boss is just rude! I think 女の子、男の子are mostly used for younger people than English "girl" and "boy".