Motion retargeting for animation characters has potential applications in fields such as animation production and virtual reality. However, current methods either assume that the source and target characters have the same skeletal structure, or require designing and training specific model architectures for each structure. In this paper, we aim to address the challenge of motion retargeting across previously unseen skeletal structures with a unified dynamic graph network. The proposed approach utilizes a dynamic graph transformation module to dynamically transfer latent motion features to different structures. We also take into consideration for intricate hand movements and model both torso and hand joints as graphs in a unified manner for whole-body motion retargeting. Our model allows the use of motion data from different structures to train a unified model and learns cross-structural motion retargeting in an unsupervised manner with unpaired data. Experimental results demonstrate the superiority of the proposed method in terms of data efficiency and performance on both seen and unseen structures.