child vs someone

child and someone both are nouns.

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

As nouns, someone is a hypernym of child; that is, someone is a word with a broader meaning than child:

child: an immature childish person
someone: a human being; person, singular, assertive existential pronoun; pronoun, person, singular; quantifier: assertive existential

Other hypernyms of child include individual, mortal, person, somebody, soul.

child (noun)	someone (noun)
a young person of either sex	a human being; person, singular, assertive existential pronoun; pronoun, person, singular; quantifier: assertive existential
a human offspring (son or daughter) of any age
an immature childish person
a member of a clan or tribe