Bring in symmetrical leaves. Have the kids fold a piece of 8½×11 paper in half, from the top to bottom. Then have them lay the leaf on the line from the crease, and trace the edge of one side. Then fold the sides back, so the child can see the tracing, and cut it out, cutting both sides out. It will unfold to look like a leaf.
Then find something similar that is symmetrical on both axes. Do the same thing with one/fourth of the object on a piece of paper that has been folded into quarters.
And lastly, try it with something from nature that has no symmetry. Contrast and compare.