There are lots of great therapies to help kids control and understand how to control anger and frustration. Play therapy, drama therapy and using their imaginations.
One of the techniques is to imagine anger as an animal (wild animals work better - like a lion or tiger). Then see themselves being calm, relaxed and in control. A time when they felt really happy about something they did or achieved. Then have them think of iron cage with strong bars and tell the tiger to go into the cage. Then think of a situation or person who made you feel angry and imagine getting the tiger in a strong cage with a strong lock, and they are the only person who has the key. Right away the tiger is calm and protected and in control. Watch it like a movie and see how quickly the anger can come under control. When you feel calm and in control of yourself and your emotions, you feel happier. It is simple but effective. Good for adults too.