> The difficulty with the plural began in Old English, where the nominative plural was at first
cild, identical with the singular, then c.975 a plural form
cildru (genitive
cildra) arose, probably for clarity's sake, only to be re-pluraled late 12c. as
children, which is thus a double plural. Middle English plural
cildre survives in Lancashire dialect
childer and in
Childermas. -
Etymonline