Little children often overhear things they are not supposed to hear, or things adults do not expect they would notice. A proverb. Be especially careful not to swear in front of little children. Little pitchers have big ears.
LITTLE PITCHERS HAVE BIG EARS
Meaning of LITTLE PITCHERS HAVE BIG EARS in English
American idioms English vocabulary. Английский словарь американских идиом. 2012