So once you're understanding what their anger is about, then what? How do you help them let go of those angry feelings?
Well, that depends. But let it go they must! And your children will need your help!
Some children (and adults) find just talking about their experience with a supportive other helpful enough to release anger, but many will also need to have some physical means of letting it go. Going for a walk, doing vigorous physical activity of one form or another - jumping jacks, skipping, dancing, push ups - or doing some pleasurable activity that can be a distraction - reading, drawing, playing with Lego - can all be helpful.
Whatever you and your child choose, the most important factor is that it MUST be HELPFUL to release the anger.
That means that AFTER they've engaged in the activity, they FEEL BETTER.
If they don't feel better - or worse, if they feel MORE angry after the activity - that's NOT a useful activity for them.
For example, some children say that they need to "punch something". You can help them try to punch a pillow - something that is soft and NOT alive, so...NOT a plant and NOT an animal. But some kids feel MORE angry after punching a pillow. In that case, this is NOT a useful strategy for that child. IF, however, punching a pillow helps, then it's a useful activity to keep in mind to help release anger. Same principle with any other activity - for physical release or distraction - you might try.
What works for your child?