Children should be seen and not heard
In the original form of this proverb it was specifically young women who were expected to keep quiet. This opinion is recorded in the 15th century collections of homilies written by an Augustinian clergyma called [John] Mirk's Festial, circa 1450:
Hyt ys old Englysch sawe: A mayde schuld be seen, but not herd.
A 'sawe', or 'saw' as we would spell it now, was a mediaeval term for saying or proverb. It has the same root as the words 'say' and 'saga'.
While the expression was aimed at women, the Old English names denoting gender are now somewhat altered. A 'mayde' was normally a young female, usually unmarried, although it was also used to denote celibate men. Girls however, could be of either sex, the term simply meaning young child.
See also: the List of Proverbs.