By default, child pages do not show up in WordPress menus... most of the time, parent pages won't show either unless you click a checkbox telling them to. Try going into "appearance"->"menus" Do you have a menu set up there? Maybe more than one?
If you do, find the primary navigation and see if you can manually add a child page. On the left side, you'll see a box called "pages" and by default it's on "most recent", which annoys me :) Change that tab to "view all" and see if your child page is listed. If it is, check it, then click "add to menu". Simply drag and drop it where you need it.
If you want it as a child to a parent link (i.e. a drop down), then when you drag and drop under the parent, drag it slightly inward (to the right) until the dashed border shows it indented.
See if any of this helps... let me know what you find.