Let me start of by saying that process based art should be the focus of your child's art for at least the first 3 years of their life. They should be allowed to explore and play with the materials involved in painting and drawing so that they will develop a confidence and love for art. I spent the first 3 years of my daughters life doing process based art. Lots and lots of scribbles, lines, colors and mixing of paint. Sometimes she would tell me a scribble was a "ghost" or "fireworks," but these images were often accidental.
At some point, around the age of 3, you can show your child how to draw a stick figure. (This would also be around the time when you might show them how to write their name.) Then keep doing process based art with them. The week after my daughter's 3rd birthday I showed her how to draw a stick figure (in purple). She surprised me by doing the pink drawing to the right. The next weekend we were at the beach and on her own she started drawing stick figures of our entire family in the sand. This was the beginning of representational drawing for her. For the remainder of that year my daughter furiously and steadfastly drew stick figures, mostly family portraits, and always stick figures. They were fascinating. I watched and never gave her any instruction.