The Association for Childhood Education International mission is "to promote and support in the global community the optimum education and development of children, from infancy through middle childhood, and to influence the professional growth of educators, and others who are committed to the needs of children in a changing society,". They provide Advocacy Toolkits and a list of Child Advocacy Organizations on the above website.
Child Welfare League of America is a great source of up-to-the-minute information on child legislation, best practice, statistics and research related to the Child Welfare System, and child advocacy issues. They have a magazine that might be of particular interest to volunteers; "Children’s Voice", it can be found at the publications link in the side bar. http://www.cwla.org/.