Often kids just want to know that it is okay to feel sad or angry. Once you hear what your child is feeling, then you need to help your child find constructive ways to deal with these feelings. If you have identified that your child is angry, you can do many activities with them to promote expression of this feeling which will help them release the feeling. Clay is a wonderful medium as it can be rolled and pushed and made into a variety of shapes to represent the person with whom the child is angry. Then the creation can be smashed, cut up, etc.
Physical activity is a wonderful release.
Together you can run, wrestle, race bikes, shoot hoops, etc. Books can be helpful when teaching young children how to accept anger. Be sure that you have read the book first to ensure that the messages are those you find meet your family values. Talk to your local librarian for suggestions. Older children can use journals or letter or story writing as a way to express themselves.