How to add childpages of parent page to widget area

Add shortcode to widget area.

function list_child_pages() { 
 
global $post; 
 
if ( is_page() && $post->post_parent )
 
    $childpages = wp_list_pages( 'sort_column=menu_order&title_li=&child_of=' . $post->post_parent . '&echo=0' );
else
    $childpages = wp_list_pages( 'sort_column=menu_order&title_li=&child_of=' . $post->ID . '&echo=0' );
 
if ( $childpages ) {
 
    $string = '<ul class="descendant-menu">' . $childpages . '</ul>';
}
 
return $string;
 
}
 
add_shortcode('childpages', 'list_child_pages');