For parents with Preschoolers:
http://parenting.uwex.edu/parenting-the-preschooler/
For parents with children of all ages:
http://njaes.rutgers.edu/pubs/subcategory.asp?cat=10&sub=69
http://www.internet4classrooms.com/links_grades_kindergarten_12/parents_homework_help.htm
http://www.internet4classrooms.com/links_grades_kindergarten_12/parents_involvement_help.htm
U.S. Department of Education: