Not in front of the children

 In the second chapter of Avot, there is a teaching by Hillel (Avot 2:5) that one should not say things that cannot be understood, if one's intention is that they should ultimately be understood. This is based on the rendering of the words שמוע and הישמע as being connected with understanding. Their original meaning is however connected with hearing, as Rabbi Eliezer Papo notes.

In his Ma'amar HaKavod Rabbi Papo explains that anyone who speaks words that are not meant to be broadcast to a wider world takes a great risk in saying them when there are small children present—even if those children do not understand what one is saying at all.  The ability of children to pick up words, phrases and names say them over again, even if it is just because they like the sound the words make when they say them, is a well-known phenomenon. 

The Talmud itself recognizes that the sometimes outrageous things children say in public can be directly attributable to what they hear their parents say in private but would never dream of articulating in public. This is found in Sukkah 56b, which relates the episode of Miriam bat Bilgah. The daughter of a Kohen, this young lady kicked the Temple Altar and shouted at it, "Wolf, wolf! For how long will you consume the Jews’ money but not stand by them in their poverty?” Miriam’s entire family was punished for this outburst. Abaye explains why: a child's words are invariably opinions that have been repeated from what has been heard at home.