The major similarity between picturebooks, graphic novels, and comic books is that all three rely on illustration to convey their story to the reader.
Although not classified as graphic novels, the popular picturebook series, "Elephant and Piggie," by Mo Willems and other similar series are great resources you can use to start introducing graphic novels to your children. Their use of speech bubbles to convey dialog alongside illustrations provides children with books formatted like a graphic novel with the support of a picturebook.