Make a classroom chart that rewards kids for every kind act they perform towards others. Write each child's name on a row with space to accumulate stickers. Each time you notice a child committing an act of kindness that is worthy of notice, whether it be sharing their toys or trying to comfort someone, reward them with a star sticker on their chart. You can also let kids report the kind acts they receive from each other, so long as they understand that you hold the final decision about whether the action qualifies as a sticker-worthy act of kindness versus an everyday expression of common courtesy. You want to reward spontaneous acts of empathy, not make children feel that being civil with each other deserves a reward.
When kids fill up their chart with a certain amount of stickers, reward them with a small treat, such as a candy bar, cheap toy from a prize box, or special privilege for the week.