To assist in the performance of the duties imposed by § 66-18, the Department shall maintain a behavioral services unit and employ as director thereof a clinically competent person. The Department shall also employ such other medical, technical and clinical personnel skilled in the diagnosis and treatment of physical diseases or mental illnesses of children as may be desirable for the operation of such unit. The personnel of the unit, when visiting the various facilities maintained by the Department for the care of children committed to the Department, shall conduct a thorough examination of each child at such facilities not theretofore examined by the unit, and other children at the facilities for whom such examination is indicated. Such examination shall be for the purpose of determining, diagnosing and treating physical and learning ailments or impairments and mental illnesses with a view to improving the general functioning of such children and hastening their rehabilitation.