No, это here can be roughly translated to "it is". Girl - it is a child. That's a very rough approximation but the thrust of it is that it is not denoting a specific child but the genre of girl in general belongs to the greater genre of children.
Am I the only one who thinks the english translation is wrong? "A girl is a child" doesn't mke sense for me. Shouldn't it be "girl this is a child" ? I am confused whether the problem is my English or Russian
I don't think it's accepting the long dash (i.e. Девочка — ребёнок) in the translation, which is inconsistent with the rest of the course so far. Unless there's a difference between - and — that I don't know about?