Emotion regulation emerges during childhood and engages prefrontal brain systems. While most developmental studies focus on the neural underpinnings of negative emotion regulation, less is known about the neuroanatomical correlates of positive emotion regulation. In adults, prefrontal areas in the left hemisphere are critical for positive emotion regulation, but whether this association is present in children is unknown. We investigated whether smaller gray matter volume in left prefrontal regions related to worse positive emotion regulation in children with and without dyslexia. Because dyslexia is a neurodevelopmental disorder of reading in which there may be greater variability in left prefrontal structures, it offers a unique window into the neural basis of positive emotion regulation. Sixty-nine children (ages 7-13) were asked to hide their feelings while watching film clips that elicited either amusement or disgust, and evaluate their emotion regulation performance. Parents reported on children's everyday emotion regulation. Across the sample, children with smaller volumes in left prefrontal regions (i.e., lateral orbitofrontal cortex and dorsolateral prefrontal cortex) had greater facial behavior (i.e., suggesting worse positive emotion regulation) yet more positive self-reported emotion regulation success during the amusement trial (i.e., suggesting more favorable self-evaluation). These regions did not relate to disgust regulation, however. Children with more positive views of their emotion regulation success also had better everyday parent-reported emotion regulation. Results suggest that positive emotion regulation in childhood relies on left prefrontal regions and a more optimistic view of one's emotion regulation abilities, regardless of one's actual emotional behavior, may confer real-world advantages.