The purpose of this work is to design a human computer interaction scheme for children in arm rehabilitation therapy. A humanoid robot demonstrates selected arm rehabilitation motions to children. Wearable inertial measurement units (IMUs) are used to monitor children's movements in order to give them proper feedback during therapy and to evaluate their motion performance. Preliminary experiments are done with 5 therapy motions which were selected by physiotherapists. Each motion is related to a goal based functional activity in order to help children to understand and perform the motion easily. Data from children and physiotherapists are recorded and analyzed. It is observed how children accept the robot and the IMUs. Preliminary results show various relationships between therapy motions and specific quaternion axes.