Many themes do not display static Page tabs in the horizontal navigation menu. Many themes that do display Page tabs in the horizontal menu do not display sub-pages (child pages) in dropdowns from that menu or in the menu bar at all. When you choose a theme like the one you have up on your blog you need to place the Pages widget in your sidebar to provide access to your pages and sub-pages. See > http://en.support.wordpress.com/widgets/pages-widget/
Currently, the Bueno, Enterprise (2 dropdown levels – Top level – Pages & Sub-Pages – Bottom Level – 10 Categories & Sub-Categories) , Greyzed, iNove, Inuit Types, Koi, Modularity Lite. Monochrome, Neutra, Notepad, Sandbox 1.6.1, Structure, Twenty Ten, Titan, Vigilance, and Vostok themes have dropdowns to child pages from parent pages in the top horizontal menu.