I've always loved old children's books, especially those from the 20's through the 60's, with their realistic illustrations in bright primary colors. Although I haven't urged them toward any one era in reading, I find that my children are drawn to these kinds of books as well. I think it's that they easily recognize the pictures as things familiar to their world, and I think it's the great colors too. There's nothing abstract about the illustrations. Besides, children's books from that time are typically happy and cheerful, unlike the moralistic Sunday School stories of the Victorian Era or the today's emotional and sentimental, and frequently even sad children's picture books.
This is a scene I see quite often: one, two, or all three of my children gathered around a book...
With pictures like this.
This book is "How Do We Know?" which looks like it was a textbook or perhaps just a children's encyclopedia-type book. I have used it in our homeschool, and they love to "read" it all on their own.