Just cut and tape the sections you cut off of one side to the opposite side. This will a create a shape that will tessellate.
Cut out a 5 cm square from heavy construction paper or tag board. Draw a shape similar to the one in the picture or, have them make their own marks.
Cut a piece from one side of the square and tape it to the opposite side as illustrated.
Repeat Step 2 for another side of your square.
Trace around the shape on a piece of paper making sure the page gets filled up with the shape they created. Then use your immagination and create interesting patterns or pictures in your shape.