One of my favorite books ever is Keys to Drawing by Bert Dodson. It is so universal, I reccomend it for all ages and stages of development as an artist. In fact, when I taught at U.V.U. I was constantly using it and when I taught at Kid's Village, I used it all the time too. It really is a great book.
I think it is full of great exercises that are explained very well and can be done at any age, parents with no art background can learn as they do the exercises with their children. Professional artists can use it to brush up on skills or just have fun and fill up sketchbooks. I love this book.