Background and objectives: Difficulties in emotion regulation (ER) have been associated with several psychiatric disorders, emphasizing a need for a greater understanding of the concept and its associations with disruptive behavior. We aimed to study the ER strategy of cognitive reappraisal with an experimental test to increase our knowledge of emotional processes in child psychopathology. Methods: In the present study, we examined emotional reactivity and cognitive reappraisal with a computer task in 160 medication-naïve children aged 8–12 comprising four groups: Fifty-eight children with Tourette syndrome (TS), 26 children with attention-deficit/hyperactivity disorder (ADHD), 19 children with TS and ADHD, and 57 typically developing controls. Results: The use of cognitive reappraisal reduced negative affect across all participants and the ability to reappraise was positively correlated with age, whereas reactivity was not. Overall, groups did not differ in reactivity or regulation success. Looking at specific differences within groups, however, only the ADHD group did not significantly decrease negative affect when reappraising. Finally, the use of strategies considered to be efficacious was correlated with regulation success, whereas the use of a less adaptive strategy related to suppression was associated with reactivity, but not regulation of emotions. Limitations: The study was limited by small, clinical contrast groups and a lack of blinding to diagnostic status in the coding of verbal strategies employed during the task. Conclusions: Cognitive reappraisal appears to be a beneficial ER strategy for children regardless of diagnostic status. Our findings indicate that children can learn and employ an adaptive ER strategy when instructed in the technique, even in the presence of attention problems, which is highly relevant to therapeutic approaches to dysregulated behavior.