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.
A great place to find Advocacy Resources for CASA Volunteers is the National CASA page for resources; http://www.casaforchildren.org/site/c.mtJSJ7MPIsE/b.5301325/k.B550/Advocacy_Resources.htm. You can access valuable, and pertinent information on everything from addiction and domestic violence, to educational advocacy and working with child welfare professionals. They provide resources by topic, a list of National CASA publications, and National CASA trainings incuding conferences, trainings, and webinars. There is also a spectacular CASA/GAL booklist by topics of interest at http://www.casaforchildren.org/site/c.mtJSJ7MPIsE/b.7799943/k.9FEB/CASAGAL_Book_List.htm.
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/.