Two experiments examined 4- to 11-year-olds’ and adults’ performance (N = 350) on two variants of a Stroop-like card task: the day-night task (say ‘day’ when shown a moon and ‘night’ when shown a sun) and a new happy-sad task (say ‘happy’ for a sad face and ‘sad’ for a happy face). Experiment 1 featured colored cartoon drawings. In Experiment 2, the happy-sad task featured photographs, and pictures for both measures were grayscale. All age groups made more errors and took longer to respond to the happy-sad versus the day-night versions. Unlike the day-night task, the happy-sad task did not suffer from ceiling effects, even in adults. The happy-sad task provides a methodological advance for measuring executive function across a wide age range.