child vs stepchild

child and stepchild both are nouns.

Word	Noun	Adjective	Verb	Adverb
child	Yes	No	No	No
stepchild	Yes	No	No	No

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

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

child (noun)	stepchild (noun)
a young person of either sex	a child of your spouse by a former marriage
a human offspring (son or daughter) of any age
an immature childish person
a member of a clan or tribe