child vs army brat

child and army brat both are nouns.

Word	Noun	Adjective	Verb	Adverb
child	Yes	No	No	No
army brat	Yes	No	No	No

As nouns, army brat is a hyponym of child; that is, army brat is a word with a more specific, narrower meaning than child:

Other hyponyms of child include babe, baby, infant, female offspring, male offspring, man-child, stepchild.

child (noun)	army brat (noun)
a young person of either sex	the child of a career officer of the United States Army
a human offspring (son or daughter) of any age
an immature childish person
a member of a clan or tribe