In my experience as a mother of a boy who had one episode of ADEM at age 6, followed by several repeated attacks of a "disease of the white matter of the brain, primarily optic neuritis," we used two different psychologists for my son from age 8 to age 12. He had behavior problems, which were worst at school (such as having difficulty starting and completing classwork, refusing to cooperate, having meltdowns, hiding under desks, etc.) and general psychological difficulties adjusting to the trauma of illness. He had an underlying learning disability that made school especially frustrating, with the ADEM on top of it. The first psychologist was okay but not as effective as the second one, though their qualifications were identical: PhD in psychology, specializing in children.
I have never heard of a specialization among psychologists for children with ADEM, and I don't think it's necessary. Only a specialization in children is needed. You will have to educate the psychologist on what your child experienced. The psychologists we worked with readily understood ADEM when we explained it to them, and perhaps they took some time to study it on their own. What is needed is a skilled counselor who can help your child process what has happened to him/her and help your family communicate well. Finding a trusted professional with good counseling skills and insights into reaching children is most important.
I would also add that there is no psychological quick fix. If I had it to do over again, I would be much more patient with my son and less eager to see that he got "fixed." My own anxiety and fear was the real enemy. The number one thing parents can do is provide a stable, loving home for your sick and/or recovering child, despite the tremendous worry you feel.
I wouldn't recommend this, but homeschooling my son for 7th grade finally broke the terrible cycle of negative school experience. It wasn't a good homeschool experience, either, but that year of absence from the pressure of school really helped my son.
My teenaged son does not see a psychologist now, but he knows one is available if he ever feels he needs help coping again. He came through the storm that ADEM created, and yours will, too.
As a side note, no psychologist or neurologist we consulted ever suggested psychotropic medications for my son. In fact, my son's main neurologist specifically advised against it, saying that he was dealing with enough medications in the way of immunosuppressants and anti-seizure drugs, and it wasn't a good idea to to complicate matters with psychiatric medications. I'm grateful we never went down that route.