G--appropriate for all
This is such a unique book. I have read fictionalized retellings of Bible stories, and I've read allegories of the Gospel. But Grandpa's Box combines the story of fictional characters--two kids and their grandfather--and retellings of biblical events. Grandpa, in an effort to give his grandkids an overview of God's entire plan for humanity, sets out to tell his them the major stories found in the Bible all from the standpoint of an epic war being fought, with each story serving as a single battle.
This unique setup does two things. First, it gives kids a good overview of Scripture from creation to the end of the world (Revelation). Second, it helps them see how God has been and continues to be in control of history. It all serves his purpose.
I read this to my 11-year-old son, and he was okay with it, though ideally it would be better for 5-9 year-olds. He did appreciate the fact that the kids in the story have grown up going to Sunday school and already know these stories, so Grandpa elaborates on them, adding details they might not have known. (The author really did a great job here! If your child has grown up hearing Bible stories they'll still learn something new. And if they haven't, they won't be lost.)
All in all, I'd recommend this for family devotions or bedtime stories for elementary kiddos.