I just came across your post. I am a drawing teacher and run an arts school at Bellandur.
The aproaches suggested by RoshMom and HiRadhu are indeed very appealing to the children and are the right tactics for children between 3 - 4 years. This is of more fun to the children.
However drawing is just like learning a language and this is a language where hand and eye co-ordination play a key role. Hence if the effort is to help the child learn to draw, try to give the child a hand drawn dotted image and have the child draw over the dots. More over simple exercises on straights lines and curved lines can be given on a daily basis. If the child is below 5 yrs of age and has just started to learn to write letters, have them write real big letters. Thankfully english alphabets are also great exercise for straight and curved lines :)
Guidance is extremely important. Hence you can teach the child to draw a house, tree, hills, clouds etc individually and then have them compose the entire drawing by themselves.
If you see that the lines are broken or crooked it shows the child is not confident about his/her drawing, in such cases you need to encourage your child really well. However if the lines are well defined even if it is slanting it shows that the child is ready for the next lesson.
Another important point is donot focus on coloring and drawing together. As children find painting easier first help them understand the right method of coloring and help them build their focus and patience at it. once you observe that the coloring is neat and is not going beyond the outlines too much you know your child is ready to receive guidance on the drawing part.
Children tend to draw very small images initially, encourage them to draw big figures which doesnt have to be perfect. The lettering practice will help them draw Big.
I think I have pretty much explained the details of the basic approach. Incase you have questions please feel free to call me at 9900159881.