There are several really good books aimed at this age group. They are meant to be read aloud to them (several times) and talked about. A good example is When I Feel Angry - http://www.amazon.com/When-Feel-Angry-Way-Books/dp/0807588970/ref=pd_sim_14_11?ie=UTF8&dpID=51btE3dscsL&dpSrc=sims&preST=_AC_UL160_SR126%2C160_&refRID=07SJD5XKNGYZ87YCXSR7
or Cool Down and Work through Anger - http://www.amazon.com/Cool-Through-Anger-Learning-Along%C2%AE/dp/1575423464/ref=pd_sim_14_6?ie=UTF8&dpID=61p82txRGPL&dpSrc=sims&preST=_AC_UL160_SR160%2C160_&refRID=1PZ6NSTVK3H6BCZNVXYZ
And you can find a few other good ones on the sites listed above.
Hope this helps.