I suppose you could just paint your computer...

A huge amount of what kids get out of art is the sheer messy coolness of it. This is why it's so important to use different materials - switching from pencils to white boards to pens to crayons to whatever else.

It's particularly interesting to take a couple of materials and see how they interact. In this picture, oil pastel was used first and then watercolor paint was put over it. When you use a bright pastel and then a dark watercolor, it looks like the lines are glowing in the dark.

Another thing you can do is write in white crayon on white paper (okay it's a leap of faith 'cause you can't really see what you're doing), and then go over it in colored crayon, ink pen, or watercolor to reveal the "secret" drawing.

So try getting out more than one medium at a time. This is something the computer just can't do for you. Unless of course you want to see how crayon interacts with LCD.