The book is designed to answer children's questions about Jesus and Christianity, without watering down the information too much. It also does not go over their heads, as long as you control how you present the book to the child. At the top of each section there is a short sentence that introduces a concept about Jesus. For younger children, this suffices as it gives them some vocabulary. Following each little bit, such as "John the Baptist said Jesus was The Lamb of God," there is a bible verse reference as well as a more detailed explanation. These more detailed explanations are more appropriate for older children. However, an adult reading this book with a younger child could use those explanations as a foundation for discussion.
My copy of the book was accompanied by a CD with 30 songs for children. Listening to that CD reminded me of my childhood. My sister and I were a part of our church's youth choirs and put on annual performances.
I would use this book with children starting in the 5-7 year-old age range. Independent readers are going to enjoy reading the information, but may come to you with questions. Otherwise, again, it is a great conversation tool. It is not a book that is meant to be read all at once.
You can preview part of the book here.