2002 study found that children with ADHD had 3-4 percent smaller brain volumes in all the brain regions measured. But children on ADHD medication had similar brain volumes to unaffected children, in some of the areas measured.

One big difference was the amount of “white matter” — long-distance connections between brain regions that normally become stronger as a child grows up. Children with ADHD who had never taken medication had an abnormally small volume of white matter.
Causes of Attention Deficit Disorder – Ben Martin, Psy.D.

ADHD and Me – Informative Video for Children and Adolescents

ADHD: Clinical Practice Guidelines for Diagnosis and Treatment – American Academy of Pediatrics