In what circumstances, if any, do we teach our kids that it is good to fight back?
There is a big difference between defending and protecting oneself from severe harm and seeking revenge. Differentiating between these two extremes is very difficult for a child. That is why Christians have maintained a standard of not fighting back. Children, by human nature, will almost always lash out at their enemy, even if they are not in any danger. They become angry and unreasonable in the face of a wrongdoing directed toward them.