We present qualitative experimental evidence that the "Wiimprovisation" process of using Nintendo Wiimotes as virtual instruments linked to a musical sound system within the framework of Group Music Therapy sessions can provide effective psychodynamic mediation for patients. We designed and implemented specific modalities for movement-based sound control and tested our tool in two clinical settings with children (age 7 to 12) suffering from behavioral disorders. Our preliminary results show immediate acceptance of, long term motivation for and meaningful expression through our system by patients. Future work will focus on assessing the therapeutic potential of our platform's personalization features in a controlled experiment.