See: https://stackoverflow.com/questions/1045439/wordpress-retrieving-post-children
See: https://stackoverflow.com/questions/3488628/wordpress-list-pages-with-children
depth
(integer) This parameter controls how many levels in the hierarchy of pages are to be included in the list generated by wp_list_pages. The default value is 0 (display all pages, including all sub-pages).
0 (default) Displays pages at any depth and arranges them hierarchically in nested lists
-1 Displays pages at any depth and arranges them in a single, flat list
1 Displays top-level Pages only
2, 3 … Displays Pages to the given depth
child_of
(integer) Displays the sub-pages of a single Page only; uses the ID for a Page as the value. Note that the child_of parameter will also fetch “grandchildren” of the given ID, not just direct descendants. Defaults to 0 (displays all Pages).