I know of no other book that helps children understand one of the most difficult concepts of Christianity - how we are made right with God through Christ's righteousness, than R.C. Sproul's book, The Priest With Dirty Clothes. Here's a brief description of the book:
Dr. R.C. Sproul tells of a young priest who falls from his horse. The mud ruins his garments, dashing his hopes of appearing before the king. But the great prince offers his own beautiful clothes, echoing the timeless message of the Lord Jesus Christ atoning for our sin and clothing us with the robe of His perfect righteousness, without which no one can see God.
I'm so thankful for theologians like R.C. Sproul who have taken the time to invest in the next generation by putting profound truths into simple language for kids. What a gift!