You can create a custom menu so your "child" pages or sub-pages appear in dropdowns below Parent Pages. A custom menu allows you to display Categories with dropdowns to sub-categories in tabs along the horizontal navigation where normally only Pages tabs are displayed. If you wish you can also include Pages with dropdowns to sub-pages and custom links in your custom menu as well. The choices of which to display are entirely up to you.
These are the how-to instructions that answer your question:
1. You create a custom menu. http://en.support.wordpress.com/menus/
2. You add the Parent pages you want to display and arrange them in the order you want.
3. Then you add and drop and drag the sub-pages (child pages) below the appropriate Parent pages, and when you have everything arranged in the menu that you want you save the menu.
4. Go to the “theme location” module at upper left on the menu page. Select your custom menu name from the pulldown labeled “primary location.” Click the save button in that module.
5. Refresh your browser so that it isn’t possibly pulling a cached page, and view see your new menu with Parent pages and dropdowns to sub-pages.