Anger management for children usually begins with basic psychoeducation. Kids who haven't had a lot of experience talking about emotions can have a hard time labeling their feelings, recognizing triggers, and connecting the consequences they've experienced back to their anger. For these reasons, plenty of time should be spent discussing each of these topics before jumping into anger management skills.
This anger management activity worksheet uses kid-friendly language to describe anger and normalize the emotion. The included activities are designed to help children think about how they behave differently when they are angry, learn about their triggers, and then come up some alternate ways of behaving when they're mad.
For another great resource on childhood anger, check out Seeing Red: