Personality variability is an important individual difference construct that is the focus of major psychological theories and relates to socioemotional functioning. Although cross-situational personality variability has been studied extensively in adult populations, little is known about variability in children’s personality. In this study, we aimed to address this gap in knowledge by evaluating whether cross-situational variability is a potentially meaningful individual difference in youth. We used a “thin slice” approach in which research assistants viewed videos of 324 children (Mage=9.92) completing 15 standardized tasks and rated youth’s Big Five personality states. Cross-situational variability in each personality state was estimated by the calculating within-person standard deviations across tasks. Results showed that (1) there is substantial variability in children’s personality states, (2) children who are variable in one personality domain tend to be variable in other domains, and (3) more variable children are described by their parents as being less competent, less agreeable, less conscientious, and more neurotic. However, associations with parent-rated external criterion were generally small in magnitude, and key psychometric properties of the thin slice personality variability index are not well-established. Our study adds tentative but promising evidence that individual differences in cross-situational personality variability are not only present in childhood, but may be consequential.