Follow the these steps to generate a list of the child pages of a current page and the further descendants (children's children).
1) While you have a parent page open, use the keyboard shortcut Ctrl + Shift + A or Menu option Insert > Other macros to select the macro for "Children Display"
2) Find and click on the Children Display macro under the "All" or "Navigation" folder
3) Check "Show Descendants" and select h4 as Heading Style. You may keep the default settings or experiment with the parameters of other fields. Press Save.
4) A list of the titles of the child pages of the current page will be generated, hierarchically arranged the way they are displayed in the Preview box. The Child Page Display is synched with any future changes you will make with child page titles and does not need manual maintenance.
See also
Confluence Documentation: Children Display Macro