I used sketchbooks with 1st- 4th grade last year. I found it to be a very good experience that I will keep developing this year.
All preliminary drawings for each lesson go into the sketchbook. Sometimes I will prepare a page to begin a lesson. For instance, we made bobblehead dolls in 4th grade. I made a hand out where the student drew the front, side, and rear view first before they started working in 3-D. This was helpful to them because some were confused as to how to use the whole 3-d form. They wanted to draw the whole figure on the front!
I also got some good feedback from parents. Seeing the sketchbooks they realized that we really do go through a series of thoughtful processes before we get to the final project!