Here is information from the American Academy of Pediatrics about how depression is treated in children and adolescents.
Many children and adolescents with depression do not receive treatment because they have never been diagnosed, they do not have access to mental health care, or they are embarrassed to seek care. Children who do not receive treatment do not function at their best, and their symptoms may worsen over time.
Treatment of depression is effective but may take a while. Following through with treatment, even when, at first, it seems to not be working, is important.
Treating depression may involve family therapy, psychotherapy (talk therapy), medicine, or combined medicine and therapy. The kind of treatment a child receives depends on the type of depression they have, how severe it is, how long they have experienced the symptoms, their age, and other factors that are specific to them and their family.