Explain to them that although we see in 3D, a camera only produces a flat image. While we may see the family dog, with the flower box behind him, what the photo may end up showing is the family dog sprouting petunias from the top of his head. If they will take a moment to visualize everything they are seeing in the lens, they will end up with much more focused pictures. Show them how moving closer or farther away by just a few steps can alter dramatically what will be in the finished photo. Being too close will often produce a fuzzy photo, while being too far back may allow too many subjects, losing the intended subject all together.
Lighting is very important.
Sun light should be behind the photographer. Even if they take a picture and they think they can see what they are taking a picture of, if the sun is in front of them, the finished photo will most likely be nothing more than a big sun blot. Lighting inside is just as important. Have them stand with lamps or overhead lighting behind them as much as possible. Make sure there is enough light, and if not, use a flash.