Mental illness in children is, unfortunately, a fairly common reason for children to receive Social Security disability benefits. The commercial website MedicineNet has very good explanations and resources for parents facing this situation.
Here is a little information from the site:
What are the most common mental illnesses in children?
Mental disorders in children are quite common and sometimes severe. About one-fourth of children and teens experience some type of mental disorder in any given year, one-third at some time in their lives. The most common kind of mental disorders are anxiety disorders, like overanxious disorder of childhood or separation anxietydisorder. Other common types of mental illnesses in childhood include behavior disorders like attention deficit hyperactivity disorder (ADHD), mood disorders like depression, and substance-use disorders like alcohol use disorders. Statistics indicate how relatively common these disorders occur. ADHD affects 8%-10% of school-aged children. Depression occurs at a rate of about 2% during childhood and from 4%-7% during adolescence, affecting up to about 20% of adolescents by the time they reach adulthood. In teens more frequently than in younger children, addictions, bipolar disorder, and less often early onset schizophrenia may manifest.
Although not as commonly occurring, developmental disabilities like autism spectrum disorders can have a significant lifelong impact on the life of the child and his or her family. Autism spectrum disorder is a developmental disorder that is characterized by impaired development in communication, social interaction, and behavior. Statistics about autism include that it afflicts one out of every 88 children, a 78% increase in the past 10 years.
