If you talk about yourself like trash in front of kids, they learn to talk about themselves that way too.

Younger kids copy tone long before they understand content. When you call yourself stupid after dropping a pass or bombing a quiz, a little brother in the room does not hear a joke, he hears the rule for what a person says about themselves. He will reach for that same sentence the first time he fails at something. None of this asks you to fake a confidence you do not have.

Watch for the next harsh thing you say out loud with a younger person nearby. Swap it for the plain version: that went badly, and here is what I will try next.