My approach with this type of issue would be to require parents to engage in counseling themselves at the same time you are working with the children. The anger they are bringing to you is natural and it can be worked out. More often than not, it is a result of some underlying irrational beliefs or feelings. It might be helpful to engage a colleague to develop an anger resolution group for parents of special needs children. I believe that a well developed cognitive-behavioral/ rational-emotive program could be just the assistant you need.