This study aimed to investigate the relationship between chronotype preference/sleep problems and symptom severity of children with Attention Deficit Hyperactivity Disorder (ADHD) during the COVID-19 outbreak and to assess the chronotype preference/sleep problems that may play a mediating role in the relationship between the reactions to trauma and severity of ADHD symptoms. The sample of this single-center cross-sectional study consisted of 76 children with ADHD and their parents. Trauma symptoms were evaluated with the Children’s Impact of Event Scale (CRIES-8); sleep habits were assessed using the Children’s Sleep Habits Questionnaire (CSHQ); and chronotype was assessed using the Children’s Chronotype Questionnaire (CCQ). There were significant differences in CRIES-8 and CSHQ scores between the eveningness type group and the non-eveningness type group. The CRIES-8 scores of children with ADHD were related to the CCQ and CSHQ scores and severity of ADHD symptoms. In mediation analyses, sleep problems were found to be the full mediating factor in the relationship between CRIES-8 scores and severity of ADHD symptoms and the relationship between CCQ scores and the severity of ADHD symptoms. Our findings indicate that chronotype plays an important role on the negative effects of home confinement of ADHD children during the COVID-19 outbreak. The role of the full mediator of sleep problems in the path from cognition to the behavior of young ADHD and non-ADHD children confined to the home environment during the pandemic period requires further assessment.