1
general::
Young children often overhear something they should not. For example, Don't use any swear words around Brian? little pitchers have big ears. This metaphoric expression, which likens the curved handle of a pitcher to the human ear, was already in John Heywood's proverb collection of 1546.
American Heritage Idioms
2
general::
Phrase(s): Little pitchers have big ears.
Prov. Children like to listen to adult conversations and can understand a lot of what they hear. (Used to warn another adult not to talk about something because there is a child present.) • I started to tell Mary about the date I had on Saturday, but she interrupted me, saying, "Little pitchers have big ears," and looked pointedly at her six-year-old daughter, who was in the room with us.
McGrawhill's American Idioms And Phrasal Verbs