Since WordPress 3.01 upgrade you can now create a custom menu that allows you to drag and drop your menu pages where you'd like them to appear, including drop downs. Go to Appearance >> Menus on your dashboard to create your custom menu.
How to create a drop-down child page in 3 easy steps:
1) Go to Page >> Add New
2) Add your title and content
3) Under Attributes on the right side of your screen under 'Publish' select a parent for your child page, and then click 'Publish'! 🙂