Well, you can't, really. Not word for word at least. You can say "эта девушка" but out of context nothing would indicate that she is your girlfriend rather than some random girl. Or you can say "Эта моя девушка", but that sounds a bit strange.
Russian just doesn't have a separate word for "girlfriend", instead substituting it with "my girl". Note that all other possible meanings of "my girl" (like, "my daugther", for example) are expressed with "моя девочка", thus leaving "моя девушка" with the only possible interpretation. It's not a literal translation, but an idiomatic one.
I make allowances for the fact the volunteers often aren't native English speakers (best that they be native speakers of the language they're teaching). Our feedback is our own contribution. "Children's composer" could exist, but it sounds awkward — "composer for children" or "children's music composer" maybe? But even that's wrong, because we use "composer" mainly for formal/classical-style music. I think "children's songwriter" would be least unnatural.
I think the most natural way to say that would be "эта девочка - юный композитор.. You need to use "девочка" here because "девушка" is not a child by definition. "Юный" means "young", "youthful" or "juvenile". In this context it's a better choice than than "ребёнок-композитор". The latter is a literal translation of "child composer" but it sounds awkward in Russian.
There is no present tense of the verb "to be" in Russian. You have to intuit it's presence, and есть is not used to express "to be".
Often, the em-dash "—" represents the present tense of "to be", but not always. (To make the em-dash on a PC, hold down the Alt key, type 0151 on the keypad, then release the Alt key.) I read in another comment that the — separtes two noun clauses, as here - "This girl — children's composer".
I believe that "She is a composer" would be Она компоситор, without any dash.
You have to be careful with есть. It doesn't mean "to be" in the same way that English means it; есть is more akin to "to exist". У вас есть literally means "By/near you (there) is/exists", which is transliterated to "you have".