You can usually get whole wheat grain from your local health food store. If you can't get any, you can order it here. You will need about half a pound for each presentation. You many want to point out the children that the grain you get from Tzivos Hashem is specially harvested for Shmurah Matzah. Allow the children to examine these grains and take notice of the fact that the grains are all whole and have no cracks. Wheat grain, which has been in contact with water (has been rained on during harvesting season) often shows signs of cracking which is caused by the expansion of the seed when it absorbs moisture. Note, in the video Let's Bake Matzos you will notice the mashgiach hand threshing some stalks of wheat to examine it for signs of cracking.