Have you tried printing in your classroom? I've done several variations of this lesson, and it's always a challenge, but we usually have nice results. I've always used a scratch foam sheet designed for kids. It has a soft surface, almost like that of a foam meat tray. A pencil, end of a paint brush, or stick can be used to "cut" your design. The problem I usually see is getting the students to hurry and make the print before the paint dries up on the foam. Also, there's the problem of the paint getting into the "grooves" resulting in poor image quality.
In this lesson, students created a mono-print self portrait, using Andy Warhol for inspiration. I introduced the color wheel; focusing on primary, secondary and complementary colors. Students drew their portraits on newsprint in pencil. I instructed students to place their drawing over the foam and secure with tape. Next, students traced their drawing, removed the paper and "deepened" their impression with a dull pencil or stick. A brayer was used to apply regular acrylic paint to the foam. Students then pressed the foam face down onto a thick quality paper, and then we mounted the printed squared onto sheet of paper. We also tried some prints on colored railroad board. This took one class period, and the foam can be reused several times-just clean off with a baby wipe. The prints above were created by a third grader and fourth grader.