Section 42. The probate court may, upon the application of the guardian of a mentally ill or mentally retarded person or of a child or the guardian of a child of a mentally ill or mentally retarded person, after notice to all other persons interested, authorize and require the guardian of such mentally ill or mentally retarded person to apply to the maintenance and education of any child or children of said ward such portion as the court orders of the estate of the ward, which is not required for his maintenance and support.