Here are some of my favorite children's books which focus on dealing with worrying, anxiety, and learning relaxation skills. Many of these will likely be available at your local public library.
If you are looking for books for parents, educators and counselors about helping children cope with their anxiety, check out this blog post.
If you are looking for apps for anxiety (for children, teens, and adults) check this post out.
Scroll down to the bottom of this page to access the free resource library!