Printmaking is such a fun medium to teach to elementary students. I try to do at least one lesson in block printing per year and it typically ends up being the favorite/most memorable for the kids.
I thought I’d go out on a limb with the fourth-graders and teach them how to do a reduction print of a sea animal. Before we began, however, I demonstrated the concept of a mirror image by having one of the students stand facing me and mirror my movements. We discussed how when I move my right hand, the student moved her left hand and vice versa. The point I tried to make was that when carving a block for printmaking (in our case, styrofoam), the resulting print will be a mirror image of that carving.
Here’s a brief explanation of the project:
Carve simple outline of sea creature. 
Apply ink evenly using a brayer. 
Press foam sheet face down on paper to make a print. 
Carve details into foam and cut out sea creature. 
Apply black ink to foam and place carefully face down on original print. 
Press, remove and voila! Reduction print.
Below is the slideshow (Powerpoint) I used to introduce the project to the students. Feel free to use and adjust it to meet the needs of your classroom.