As others have mentioned, there are many modules out there that manage node-to-node relationships. One thing that was a problem the last time I looked was how to go about restricting access to node/add/[type that requires parent]/[nid of parent] without other things breaking. Previous discussions led me to override the menu settings to disallow access to this path, but since the menu system does not prioritize it's entries, you may find that your modification of the menu permissions doesn't always work. The result is that the node/add page shows links to add content that is not allowed to exist without a parent. The Node Relativity module (http://drupal.org/node/17004) essentially does what you're looking for, but hasn't been updated in a while. It is big and needs some refactoring and is probably a heavier solution than you're looking for, though.
what about simply removing the node/add page using menus module? that page is often worthless anyway (as is the whole navigation block IMO). -moshe