What the word covers now
In present-day English, child does two jobs that speakers rarely bother to separate. It names a young human who has not yet reached adulthood, whether that line is drawn at puberty, at a legal majority, or by custom. It also names a son or a daughter at any age: a person of forty is still a child of their parents. Older uses sit at the edges of the same noun. An unborn infant can still be called a child, and the idiom with child still means pregnant. In looser speech the word can mark immaturity in an adult, or a product of a time or place.
The origin of the word child is not a borrowing from French or Latin. It is a native Germanic noun that English has carried since the Old English period, and the modern breadth of meaning is later work.
From Old English, not from Kind
The immediate source is Old English cild (also written ċild), already in use before 1150. In that period it was usually a strong neuter noun, and its core sense was narrower than the school-age meaning most speakers now take as basic. The word pointed to a fetus, a newborn, an unborn or newly born person. Early uses sit close to the mother, as if the noun first named the fruit of the womb rather than a stage of growing up.
The path is inheritance, not loan. Middle English kept the same stem, and modern English kept it again. German Kind and Dutch kind look like cousins on the page, but they are a different word. Those forms go back to a prehistoric participle meaning born, related to the Latin family of gens and the verb behind genus. English held on to a rarer Germanic item instead of that common continental noun.
Old English had another word for offspring, bearn, which survives in Scots as bairn. For a time the language therefore had a clearer split than it does now: one noun for the newborn body, another for a parent's young. Child later took over both roles.
A womb-word with a thin family
Behind Old English cild stands a Proto-Germanic form reconstructed as kelþaz or kiltham, with a West Germanic stage kilþ or kelþ and a meaning in the region of womb and fetus. The most secure match outside English is Gothic: kilþei for womb and inkilþō for a pregnant woman. Old English itself had the compound cildhama, a child-home, for the womb. That cluster is the best evidence that the English noun began as a birth-and-body term, not as a general label for a young person.
Beyond Gothic the picture is less settled. Many accounts list Danish kuld and Swedish kull, both used for a brood or a set of offspring from the same parents, as Germanic relatives. Other authorities treat that Scandinavian link as disputed and say the English word has no further secure cognates inside the family. Links outside Germanic are thinner still. Proposed ties to Sanskrit words for belly or womb, to Gothic kalbo (calf), and to Latin gleba (a clod of earth) remain uncertain. There is no agreed Proto-Indo-European story that every handbook accepts. The honest account stops at a Germanic womb-and-fetus noun with one clear Gothic parallel.
Why children is a double plural
The form children is not a regular English plural. It is a double plural, built in two historical layers.
In early Old English the nominative and accusative plural of cild was often identical to the singular: one cild, several cild. Around the late tenth century a distinct plural cildru (genitive cildra) became common. That -r type belonged to a small class of neuter nouns for the young of a species, the same class that once marked lamb and calf. The extra ending was likely analogical, a way to make number unmistakable once the old unchanged plural had grown unclear.
In early Middle English, southern dialects then added the weak plural -en, the ending still visible in oxen and in brethren. The result, children, stacks -r and -en. Northern and north-midland speech kept the older r-plural without -en. Dialect childer and the feast-name Childermas preserve that stage.
The vowel split between child and children has a separate phonetic cause. The stem vowel lengthened before ld in the singular but not before the ldr cluster of the plural. After the Great Vowel Shift, that length difference became the familiar contrast of a long diphthong in the singular and a short vowel in the plural.
How the sense widened
Late Old English stretched the word from infant toward a young person before puberty. In the same late period it could mark a youth of gentle birth, a use that the later spelling childe kept as a kind of title. From the late twelfth century it also meant one's own offspring, the sense that bearn had more often carried. The phrase with child, from the same century, did the opposite of widening: it froze the old fetal sense inside an idiom for pregnancy.
Between the sixteenth and seventeenth centuries the word could, in some contexts, mean specifically a girl. Early modern drama even uses child in contrast with boy when a newborn's sex is in doubt. Figurative uses — a child of a place, of an age, of a quality — appear from the late fourteenth century. Later senses, such as a legal minor or a dependent data node, sit on top of that older split between age and descent.
Most Indo-European languages use one word for both a young human and a parent's offspring. Latin is a well-known exception, with separate terms for children as young people and children as sons and daughters. English once had that split more clearly. Over time child absorbed both jobs, while bairn receded to regional speech.
A small surviving clue
If you want a single reminder of where the noun started, keep the Gothic womb-word and the English idiom with child in the same frame. Standard English now uses child for school-age years and for adult sons and daughters alike, but the older body sense never fully left. It is still there in pregnancy talk, in childbirth, and in the fact that the most trustworthy relatives of the English noun are words for the womb, not words for a boy or a girl at play.