Materials:
-Paper
-Pencil
-Eraser
-objects to draw
I was horrible at drawing when I was younger. Well, to be more specific, I was (and am) bad at drawing things in correct perspective to other things. A lot of people have trouble with this. Say you have a cup, a ball, a shoe and a vase all grouped together. How do you draw them together so they're the right size in relation to each other?
My problem is that I would draw each object seperately and not work on the objects as a whole group. I would draw the cup completely first, and then move onto the next object. And the ball would be way too tiny and the cup would be too big, and it wouldn't look right. What you want to do is to not focus on each thing as an individual object but see the group as a whole image together. The objects are related to each other--they cast shadows; depending on the materials, you can see through one thing to another, one will be so many inches taller than that one, etc.
It's easy to get caught up in the minutiae, the little details, before getting the general shapes out of the way.
First, you want to pick simple objects that your kids won't get frustrated drawing. And you want at least three or four things together to make an interesting drawing. For example, a sippy cup, a ball, an apple and an open book would make a nice still life. Make sure that there's some height difference amongst the grouping (ex: a tall thin cup with a squat tomato).If you want, put a strong light source on one side to make some nice shadows.
Before your kids start drawing right away, have them take a moment to really look at the objects in front of them. This is something I learned in college--we tend to draw things from memory rather than looking back at the still life in front of us. We draw how we think it looks not necessarily how it really is.
Next, have them figure out which is the smallest object in the group. This will be what you measure all the other objects with.
I'll explain what I mean: You want the objects to be to scale to each other. Say you have the sippy cup, ball, apple and book set-up I mentioned earlier. The cup is the smallest object in the group. Hold out your pencil at arm's length (think of the classic image of the artist with one eye closed and holding out their drawing pen). Close one eye, and measure the cup, starting at the top of the pencil, slide your thumb down the pencil to where you see the bottom of the cup. That is your standard unit for measuring. Keep your thumb on that same spot on the pencil and look at the other objects now. The ball is two cups high and two cups wide; the book laying on its side is half a cup high and three cups wide; the apple is one and a little bit more cups high and about the same width. So now when you draw the cup on the paper, you can make sure that the other objects relate properly. It doesn't matter how big the cup is on the paper, just draw the other things that much bigger. Oh, I drew the cup three inches tall, so the ball is six inches tall, etc.
When you start drawing the still life, don't make super dark lines right away. At this stage, you want to get the basic shapes down first. Make a quick, loose sketch of the the overall image. Once you have all the objects down, you can put all the details in. A successful drawing is like building a house--you need to have a good foundation to build the frame on. If the objects aren't in the correct place in comparison to each other, it's hard to get the details right later.
Once all the shapes are down in simple lines, start filling in the details. Where are the brightest light spots? Where are the darkest shadows? Do any of the objects reflect light onto another in the grouping? Where do the shadows fall on the table? Part of drawing is learning to really see the things in front of you, taking note of the general shapes of objects to the small details you wouldn't normally notice.
This takes practice, just like any other skill. The more you draw, the better you'll get.
If you feel overwhelmed or unsure of how to go about learning or teaching drawing skills, I suggest reading Drawing on the Rigth Side of the Brain by Betty Edwards. You can find some more info about it and reviews of the book here.