Mental illnesses affect about 20% of children in the US each year. Some of the most common disorders seen in children include ADHD, which impacts 3-5% of school-aged children and is characterized by inattentiveness and hyperactivity; oppositional defiant disorder, seen in around 15% of children and defined by hostile behavior toward authority figures; and mood disorders like depression, which can cause symptoms such as persistent sadness, irritability, and loss of interest in activities.