First find the page id using page title or page page slug.

For post title use following code:

<?php
$page = get_page_by_title( 'About' );
wp_list_pages( 'exclude=' . $page->ID );
?>

For post slug use following code:

 $page = get_page_by_path( 'your-page-slug'
echo $page->ID;

Now List the child page by post slug

if ( $page = get_page_by_path( 'your-page-slug' ) ){
  wp_list_pages( 'orderby=name&depth=1&order=DESC&show_count=0&child_of=' .$page->ID . '&title_li=' );
}

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