Children should be seen and not heard

From Spanking Art
An poem for instructing children in table manners
Hyt ys an old Englysch sawe [saying]: ‘A mayde schuld be seen, but not herd.’
  — [c 1400 J. Mirk Festial (EETS) I. 230]

Originally directed at (young) women but generalized to all children, the proverb children should be seen and not heard deals with the a belief that children should not disrupt the adult world when present in the background. Although modern usage is not to be taken as sweeping as the literal meaning, it does cover things such as manners such as saying excuse me then waiting to be acknowledged when they want to interrupt a conversation between adults.

It also covers when the children's world is a disturbance; but this is less agreed upon.

The phrase also exists in other languages, such as German: "Kinder soll man sehen, [und/aber] nicht hören".