How does empathic physiology unfold as a dynamic process, and which aspect of empathy predicts children's kindness? In response to empathy induction videos, 4- to 6-year-old children (N = 180) showed an average pattern of dynamic respiratory sinus arrhythmia (RSA) change characterized by early RSA suppression, followed by RSA recovery, and modest subsequent suppression during positive resolution of the empathic event. Children's capacity for this pattern of flexible RSA change was associated with their subjective empathic feelings, which were concurrently associated with more sympathetic and prosocial responses to others. Conversely, only children's dynamic RSA change longitudinally predicted prosocial behavior 2 years later. These findings have implications for understanding the dynamic and multifaceted nature of empathy, and its relation with prosocial development.