Build parent and child page tree - https://maheshwaghmare.com/?p=37686

parent-child-page-tree.php

<?php
/**
 * Build the nested page tree
 *
 * @todo Change the `prefix_` with your own unique string.
 *
 * @param array $childs.
 * @param int $parent_id.
 * @return array
 */
if( ! function_exists( 'prefix_build_tree' ) ) :
	function prefix_build_tree( $childs = array(), $parent_id = 0) {

		$tree = array();

		foreach ($childs as $child_id => $single_page ) {
			$single_page = (array) $single_page;
			
			if ($single_page[ 'post_parent' ] == $parent_id) {
			
				$children = prefix_build_tree( $childs, $single_page['ID'] );
				if ( $children ) {
					$single_page['children'] = $children;
				}

				$tree[] = $single_page;
			}
		}

		return $tree;
	}
endif;