Query for child nodes
Out of the box, there is no concept of sub pages or child nodes in Drupal. What you refer to is the menu structure used for navigation. A node can have a menu items linking to it and menu items can be organized hierarchically. The system was not designed to create and define relation between nodes, but only for navigation. If you look at the API, there is no simple way to retrieve the children of a menu item.

The book module included in Drupal core can be used to create a hierarchical structure for nodes. There are other options such as the relation, references and node hierarchy modules.