Although children display strong individual differences in emotion expression, they also engage in emotional synchrony or reciprocity with interaction partners. To understand this paradox between trait-like and dyadic influences, the goal of the current study was to investigate children's emotion expression using a Social Relations Model (SRM) approach. Playgroups consisting typically of four same-sex unfamiliar nine-year-old children (N = 202) interacted in a round-robin format (6 dyads per group). Each dyad completed two 5-minute tasks, a challenging frustration task and a cooperative planning task. Observers coded children's emotions during the tasks (happy, sad, angry, anxious, neutral) on a second-by-second basis. SRM analyses provided substantial evidence of both the trait-like nature of children's emotion expression (through significant effects for actor variance, multivariate actor-actor correlations, and multivariate intrapersonal correlations) and the dyadic nature of their emotion expression (through significant effects for partner variance, relationship variance, dyadic reciprocity correlations, and multivariate interpersonal correlations).