I've just stumbled upon this site that publishes, in Greek and English as well as several other languages, Orthodox books for children. Friends gave our sons one of these books several years ago, My Warrior Saints and it has remained a favourite not only for the story but also for the wonderfully vivid and lavish illustrations. As a parent I am always on the outlook for books like these, some examples of which I have placed below.
My Synaxarion 1 - January (Orthodox Coloring Books). (The publisher has a whole series of these, January-December. The link above will take you to the Amazon page with the rest of them.)
My Synaxarion of the Holy Apostles;
Orthodox Children's Illustrated Lives of the Saints Book/CD Set St. Demetrios;
The "Creed" in Coloring Icons (Orthodox Coloring Books).