A parent who models kindness consistently in ordinary, unremarkable moments, patience with a frustrated shop assistant, a genuine thank you to a delivery driver, tends to teach a child more about kindness than any number of direct conversations specifically about the importance of being kind to others.
Children are unusually attentive to the gap between what adults say and what adults actually do, which means inconsistency between the two tends to undermine a lesson about kindness far more thoroughly than simply never mentioning it at all in the first place. The benefit compounds quietly, well past the point most people stop paying attention to it.
The most effective teaching, then, tends to happen when nobody is deliberately trying to teach anything, ordinary kindness demonstrated as a routine part of daily life, absorbed by a watching child gradually and almost entirely without either party being consciously aware it is happening at all.