I grew some decent crystals once when I was younger. Perhaps you did too? It’s an excellent (and cheap) science project for a young person to do, so do encourage them to have a go, whenever you get the chance.
Whatever you do, perhaps it is not a good idea to show them these giant selenite (gypsum) crystals first:
Fantastic aren’t they?! (Click for larger view.) They are in some truly extraordinary caves in Naica, Chihuahua, Mexico (the “Cueva de los Cristales”). You can learn more about them, and recent thoughts about the conditions needed for their formation in their underground location, in this BBC article. There’s also much more here at the Naica Crystal Caves website, including several more images.