Drawing revolves around five basic skills; edges, space, lights and shadows, relationships between drawn items, and gestalt (putting it all together). Encourage children to draw with their eyes on the object they are drawing. Try putting a square of paper on their pencil, above the spot they grip, so that they cannot see the line they make. Have them draw practice lines first. Have them practice drawing each part of the shape separately.
All of these aspects are intertwined, as kids gain vital skills through practice with tools such as crayons, pencils, markers, and paintbrushes.