Is there a treatment for brain damage caused by overdose of anti-psychotic medication taken as a young child?

Likely no: As you can imagine, this is a very general question without any details regarding the actual medication given, the medication effects, the patient's psychiatric disease, nor any imaging or psychologic exam findings. In spite of the general nature of the question, it is fairly safe to say that any damage potentially caused by medications as a child would be irreversible in adults.