One of the most important things is encouraging your child. Do not criticize or pressure. Art should be a fun process. Contact your local museums and find out if children's art classes are offered. Contact your school and find out if the PTA or a Parents Volunteer Program offer afterschool programs. Here are some good sites for children learning how to draw. Parents should supervise all websites with younger children.
Billy Bears Coloring Book and Drawing Book
About Drawing and Sketching
Mark Kistler Drawing in 3-D