If you interpret kindness as simply giving the other child the first turn with a toy, you have missed the essence of kindness. Sharing toys so that both can have equal time with a toy is not, by itself, biblical kindness. Sharing this way may result in a more pleasant day, but what will the child really be taught? Could she be learning, "If I am kind I will get what I really want. My brother won't fuss, I get to play with the toy, and Mommy is happy with me for being kind."” This is a recipe for teaching selfishness. This child is being trained in a subtle form of self-service masquerading as kindness. Her kindness is not sacrificial; rather it is a tool that is achieving a more pleasant life for her.
Yes, kindness is wise behavior; but there is much more to wisdom and kindness than first meets the eye. The apostle Paul describes what true kindness looks like in Ephesians 4:32:
"Be kind to one another, tenderhearted, forgiving one another,you as God in Christ forgave you."