Introduce them to the various types of materials which can be used for drawing including pencils, charcoal, crayons, markers, chalks, pens, brushes and inks.
Ask them to initially choose one of these and explore the types of lines, which can be created.

Can you create thick and thin lines?
Can you create zigzag, wobbly, wavy, straight or other lines?
Can you use one type of line to create a pattern?
Choose a different type of paper to work on. Does this change the look or feel of the drawing?

Choose a different drawing tool and explore how this changes the types of line you create.
Select your favourite background paper and your favourite drawing tool and record all the different types of lines you have discovered. Display this or include in your art folder.

Follow up.

Ask the children to do a doodle drawing using one type of drawing tool.
Fill in the spaces and shapes that are created using colours or patterns.
Show some reproductions of cave paintings and discuss them.
Look at a line drawing by an artist. Discuss the kinds of lines the artist created and the materials s/he used.
Look at drawings used to illustrate books or at cartoons.
Use a computer graphics programme to create different lines.