Thomas Nelson provided for me the book God's Promises for Girls, by Jack Countryman and Amy Parker. This is a devotional-type book designed for young girls. My 8-year-old daughter read it with no problem and it seemed to be just right for her reading level.
Beyond readability, of course, is the content of the book, and it's very well done. It divides each day into two pages (one page spread). For each day, there is one topic (example: Trusting God) and a few verses to meditate on. There is no commentary; just the verses, plus some very nice illustrations.
I highly recommend this book for girls around age 8, give or take a few years. In fact, as an adult male, I found I could get a lot out of this book as well.
Note that there is also a boys' version of the book which I did not review.