The common childhood anxiety disorder is Separation Anxiety (SA). As the name implies, the anxiety of SA features fears of being away from people, including family and sometimes friends, to whom the person with SA is close.
Manifestations of SA include difficulty sleeping alone, nightmares of harm to the person with SA or people close to them, distress when parents or siblings do not come home when expected, and difficulty leaving home and family to go to school or even the inability to go to school unless there are siblings or close friends at school (in which case the person with SA moves between friends and siblings, careful always to have someone they are comfortable with close to them). It is thought that SA often becomes GAD in adulthood. Think of the possibility of undiagnosed SA in young adults who have trouble leaving home to live independently.