My parents made many smart decisions about how their kids should behave. They were the opposite of mellow in many respects, but they approached parenting calmly. They were also older when they had their first kid (29 and 36, which was almost elderly in 1951). That maturity may have been their guide.
Their best decision? If we squabbled, my mom would say, "Oh, brothers and sisters don't fight." Then she'd point us to a way to resolve what we were arguing about.
A child psychologist might say she reduced our ability to resolve conflict, but that would be incorrect. We all talk first, no matter the life situation. And we five are close friends.
Since I can't clone my parents' philosophy (or insert it in the parents of political leaders) I've done what I always do when I have a problem. I look to books.
Here are some articles or books about helping children resolve conflict and, more broadly, talking about what peace looks like.
Please feel free to put other examples in the Comments section.
Peace be with you.
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