In peaceful "primitive" cultures, kindness is sustained from generation to generation by a kind of "trickle-down" effect. At its core is the commonly held value of serving and delighting younger people, especially babies.
Adults appreciate and support the delight of adolescents,
who delight in the joy of prepubescents,
who enjoy entertaining younger children,
who love to carry babies and play with toddlers.
The elegance of this top-down, pleasure-oriented value system is that the youngest people receive the most (and give the least) at the developmental stage when they are naturally narcissistic, while those who give more are more adept at deriving joy from giving.
Adults in our culture often fear adolescents,
who call prepubescents "dweebs,"
who disparagingly call younger children "babies,"
who compete with real babies for scarce love and attention.
When your children behave unkindly, remember that you can't enforce authentic kindness. Instead, let it trickle down by modeling unconditional generosity. Using your creativity, find a way to serve and delight both "aggressor" and "victim" ...and yourself, too!