Attention-deficit hyperactivity disorder (ADHD) is the most frequently occurring pediatric neurobehavioral disorder. Although emotion reactivity and regulation are frequently impaired in ADHD, few studies have examined these factors in preschool aged children with ADHD, and none have explored the neural correlates of emotion reactivity and regulation in this group though event-related potentials (ERPs). Children aged 4 to 7 with (n = 24) and without (n = 30) ADHD symptoms completed an attention task composed of four blocks: baseline, frustration, suppression, and recovery. In the frustration and suppression blocks, negative affect was induced by false negative feedback. During the suppression block, children were asked to suppress emotional expressions. Children in both groups reported increased frustration from baseline to the frustration block, but the magnitude of the increase was significantly larger for children with ADHD. Both groups showed similar increases in observed expressions of negative affect from the baseline to frustration block, but children with ADHD expressed more negative affect in both blocks. In the left frontal and frontocentral regions, typically developing children demonstrated enhanced P3 amplitudes during the frustration block, suggesting that these children were able to allocate greater attentional control in the face of an emotional challenge. In contrast, children with ADHD symptoms did not show significant P3 enhancement during the frustration block. During the suppression block, children with ADHD demonstrated smaller reductions in self-report and observed expressions of negative affect compared to typically developing children. Typically developing children continued to demonstrate enhanced P3 amplitudes in frontal and frontocentral regions during the suppression block, compared to baseline, but children with ADHD did not. This pattern suggests that preschool aged children with ADHD are not as effective as their peers in suppressing emotions and engaging top down attention mechanisms. The present study extends a growing body of literature that suggests that emotion dysregulation is a central component of ADHD already present in the preschool years and underscores that emotional contexts may exacerbate attentional deficits in ADHD.