Hiding child pages till parent is clicked on

How do I make it so the sidebar on my WordPress only shows the child pages when the parent page is being viewed?

So, for example, pretend I was viewing my homepage and my site is about countries, on my sidebar it would show:

Asia
Africa
Americas

Then a visitor clicks the Asia link, and once the visitor reaches the Asia page the sidebar then shows:

Asia
–India
–China
–Japan
Africa
Americas