Here is a great tutorial that both young and old can do, by the talented Arounna Khounnoraj for Bloesem Kids.  The pretty water colour and crayon resists each other and makes for a colourful piece of art to frame or just peg on your childs wall.

What you will need:
*watercolour paper or heavy stock paper 8 x 10 in. 
*crayon (white is nice – I used grey) 
*watercolour paints and brushes 
*cup for water 

Step one: 
..Take the crayon and do a drawing on the paper 
Step two: 
..Cover the entire paper with just water (this is a wet on wet method) 
Step three:
..Take the watercolour and paint over the entire surface – the crayon will resist the paint and stand out nicely showing the drawing.

Let stand to dry.