Niña refers to a female child.
Chica and muchacha are more or less the synonymous, they refer to a youngish female, could be a child or a young adult.
The reason it's translated as girl here is because we use "girl" all the time in English to refer to young women even though that is not technically correct, since girl literally refers to a child.
I think you're right, and I did report it. "Gal" should be accepted as a translation for "Chica." My Mexican-born native Spanish-speaking spouse uses chico as "guy" with the analogous chica as "gal," in the sense of "guys and gals." While idiomatic, I think it should still be an accepted translation here.
My native-speaking girlfriend (Mexico City) says that niña is young girl, chica is young teenager (through maybe 18 or 19), and muchacha would be young woman (maybe late teens to mid twenties.) These age ranges are not precisely defined and might depend on the person or context.
Even though I am no "experienced Spanish speaker", I'll go ahead and answer your question (you may doubt the validity of my answer, of course): I don't think "the kid" is a valid translation for "la chica" but for "la niña". "La chica" is more used like "the chick", "the young woman" or even "the babe", so it's mostly in a sexual context at least a bit. As you wouldn't call your own 10-year-old daughter "chick" you wouldn't call it "chica", either. Hope that helps.
That sounds like it would make sense. But I was watching a Spanish movie where children were being called into the house. "Ven aqui, chicos." I also heard the term chiquitico (not sure of spelling) as I was growing up in Miami. I got the impression it meant "little child." I hope some native Spanish speaker can set me straight.
Here in a heavily hispanic area in So Cal, I have heard women refer to other grown women as "chica", usu. the connotation at the time was that all of these women were well known to each other. I've also heard (mostly) women refer to their own daughters, mostly preteens, as "chica". I almost NEVER hear anyone say "muchacha" here. If I hear it, it is usu. on TV. When I very rarely hear someone say "chico" it seems to be with condescension. "Muchacho" is also mostly heard among compatriots, good mates, or with giggles about some man behind his back. Mostly what I hear here, referring to females of any age, is the much more affectionate "Mija" (Mi hija). Occasionally, I hear some woman call to her daughter with "Nina".
Gee, I think I've accidentally picked up a very few words of Spanish since coming to So Cal.
By my reading of these (del Diccionario de la lengua español de la Real Academia Española:)
they are essentially synonyms (along with muchacha.) It does seem to me that chico/chica is shaded in meaning towards size and niño/niña towards age in the given definitions. As always context is king and I believe there are regional variations in how the terms are used.
What I learned in reading on this is that chico/chica seem to NOT be considered slang terms by native speakers (which is what I believed before.) If I saw "The Girl" without context I would more likely translate it as «La Niña» than as «La Chica» but apparently either would be correct.
[Edited to fix the niña link]
I wrote "the young woman," because "chica" does not suggest that the person is a child, but a young woman. But it was marked wrong, even though it seems this would be the closest accurate translation for this phrase that is not slang such as "chick" or whatever. "Young lady" and "young woman" should be acceptable translations for "chica," yes?