Without actually being proficient in Russian (so please, if someone more knowledgeable than me knows better, correct me), it seems to me like adding the "это" makes it into a more general statement about what girls are. You're saying that girls are children, while saying just "Девочка - ребёнок" could mean "The girl is a child" as in a particular girl. From what I've gathered, since there's no "the" or equivalent in Russian, they would use things like word order, or maybe things like this to convey those nuances.
It is actually an irregular plural for ребёнок. I wouldn't believe it at first either, but all the Russian speaking people have assured me it is so, and the dictionary searches appear to support this statement.
For information on words and the conjugation and other such words in English, here's a handy dandy website Theron passed me when I ran into this exact same issue.
You can fill out whatever word you are curious about, and it will open a new window with morphological information about it. This means, all cases are shown. It does not, however, take the accented letters, so instead of ребёнок, be sure to use ребенок to prevent it from throwing an error at you.
You'll note the page is showing both singular and plural, and that дети is shown as its base pluralised version, with the ending adjusted for each of the causes. (If you've not yet run into the causes yet, don't worry, it'll show itself in time. And when it does, you'll have this website to make your life a whole lot easier. :3 )
I have to ask something just to make it clear, is ребёнок a boy or the child in the general (without knowing what is the gender of the child) and is ребёнка a girl? Is that the reason why this cannot be translated like "the girl is this child" because that sentence would be like "девочка - это ребёнка"?
Ok a I just saw that I did make mistake because the word ребёнка means the same as ребёнок but it's just in the different case... sometimes it's not good to think in different language and to take a logic of different language to learn yourself Russian lol ( I'm Serbian and we have a similar language like Russian)
This sentence left me gobsmacked. I started to wonder if it was functioning as a topic marker similar to Japanese は, and whaddya know it on the wiktionary page for это is a note: "One of the uses of this form is to connect subject and predicate as kind of topic or subject marker" which also clarifies with an example about soy milk that the это is optional in this use case.
As has been noted elsewhere, word order is used to convey emphasis in Russian. Thus, one way to interpret the structure "x - это y" may be "as for (a) x, it is (a) y," or, to use a more natural but purely orthographic English equivalent, "x: y"
But don't translate it that way or Duo gonna wreck ur day