Child

Old English cild meant a child, a young person. The word is ancient: it has cognates in Old High German (kind), Gothic (kilthei, womb), and appears to descend from a Proto-Germanic root related to the concept of something carried in the womb. Old English pluralized cild as cildru — the -ru suffix was the plural marker. Then English added another plural suffix, -en (from Old English -an), to make cildren. Modern children is a double plural.

The double plural of children is one of the most elegant fossils in English grammar: you can see the language adding a new plural ending onto an existing plural form, not quite trusting the first ending to do the job. The same thing happened to brethren (from brother), which added -en to an already-plural form.

Old English cild had a specific secondary meaning: a young noble who had not yet been knighted. A child was someone in training, someone not yet fully actualized in their social role. This is the Child of 'Lord Randal, my son' and other ballads — the young nobleman, not merely the small person.

Children were not, historically, a protected class in the way modern childhood assumes. The concept of childhood as a distinct and protected stage of life, with its own psychology and rights, is largely a product of the 18th and 19th centuries. Philippe Ariès's 1960 historical work argued that medieval Europe had no concept of childhood at all — children were treated as small adults. The word is older than the concept it now names.