Empathy is a key component of socioemotional skills and contributes to prosocial responses and relational closeness. There remain challenges to accurately assess children's empathy, particularly given the reliance on external observer reports, and the lack of frameworks for how empathy may manifest differently at various developmental stages. In this study, we systematically reviewed research on the measurement of empathy in children ages 3-12. We identified 193 independent research publications reporting the use of empathy measures with children. From those, we identified 136 different empathy assessments across studies, with 66% involving rating scales and 48% involving self-report data. We conducted a reliability generalization analysis and meta-regression to examine the psychometric properties of empathy measures for children. We then discuss implications and recommendations for future research.