Objective: Unintentional injuries, the leading cause of pediatric mortality, are caused by a complex set of intrapersonal and environmental factors. The role of three critical variables - parental supervision, children's temperament, and estimation of children's physical abilities - was examined. Methods: Sixty-four 6- and 8-year-old children completed a laboratory experiment with a parent. Both children and parents judged the child's ability to complete reaching, stepping, and crouching tasks. Parents also completed a parent-report measure of children's temperament. Results: Both children and parents overestimated children's ability, although children did so more than parents. Parents of temperamentally impulsive and undercontrolled children judged that their children could complete tasks that were actually beyond the child's ability. Temperament also affected children's judgments while parents were known to be present or absent: Temperamentally impulsive and undercontrolled children were more accurate in their judgments when parents were standing next to them than when parents were hidden from view behind a one-way mirror. Conclusions: The mechanism by which parental supervision might protect children from injury appears to be at least two fold: (a) Parents overestimate children's ability less frequently than children themselves, suggesting supervising parents could intervene to prevent children from attempting dangerous activities; and (b) children judge their physical abilities more cautiously when parents are present. Implications for temperament theory and for injury prevention are discussed.