Backyard Horses was exactly the series I was looking for for my daughters, who love Jesus, love horses, and are in 2nd grade. Miss Mackall also wrote a series for older readers called "Winnie the Horse Gentler" and I tried that first but I'd recommend it for middle school students and up. Besides being harder to read, those books sometimes deal with harder issues that wouldn't be understandable by anyone younger than pre-teen. Backyard Horses, however, is a fairly new series and is written at around a 2nd/3rd grade reading level. Love, love, love this series!
My daughters love Ellie, the heroine of the story! Ellie is a sweet, oftentimes feisty, day-dreaming 4th grader. In the first book, she often day-dreamed about her own horse. She did a "science" experiment... begged, cried, and prayed for a horse of her own. God answered that prayer and brought her a palomino horse she named Dream, hence her new horse's name and this books title "Chasing Dream". Because it's a continuing story, I recommend reading the first and second books but you definitely don't have to do that in order to enjoy Chasing Dream.
I highly recommend this book to younger children. It's filled with a good story, important lessons, and a little girl who trusts in God. I highly recommend Chasing Dream as well as Horse Dreams, Cowboy Colt, and Nightmare, the others in the series.