I've had the opportunity to read a couple of chapters from this book by Bruce Ware. He is a Theology professor at Southern Baptist Theological Seminary. If you are looking for a good book to go through with your kids during the summer this one of my recommendations. It is a systematic theology book, but is written in a way that is easily understandable for children. I had the privilege of reading portions of it to my children this past weekend and they said it was understandable and they loved the way he explained the doctrine.
