Programming tools like Logo and HyperCard give children a chance to be creative, like writing and drawing do.
Beyond self-expression, children could learn the basics of
* interaction design ("'Click here' doesn't explain what a button does"),
* debugging ("It's okay to be wrong at first. I can just keep fixing it until it works")
* and geometry ("I can make a shape look more like a circle if I give it more sides").
EDIT: Here's a neat example of HyperCard's power: http://www.loper-os.org/?p=568. It's a bit complicated for a child, but it gives you a sense of HyperCard's potential.