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');