Those descended from or contained within another. Named from the parent's side, which is the whole of the relation.
Old English cildru, an unusual double plural of cild (child) that picked up the regular -en ending on top of an already-plural form; see child.
The word carries no claim about a child's own nature, only about where it sits — something counts as a child by virtue of what holds or descends it, and asking what it 'is' apart from that relation asks a question the word was never built to answer.