I have very good idea. How about making a periscope. Its very easy, and takes very less time. Its perfect for a 4th grader using mirrors. What you do is take two mirrors and build a 3d rectangular box, and attach each mirror at its end (inside). And make two wholes right next the the mirror. Periscope helps us to see from one side of it.
For more information about its making procedure >>> www.sciencetoymaker.org
For more information about its making procedure >>> www.sciencetoymaker.org