WordPress sub-pages menu

Using wp_list_pages() or the built-in Pages widget, you can use CSS to show only the current top-level page and its children. Here is the CSS:

#menu_id h1,
#menu_id li {
 display: none;
}
#menu_id li.current_page_ancestor,
#menu_id li.current_page_ancestor li,
#menu_id li.current_page_parent,
#menu_id li.current_page_parent li,
#menu_id li.current_page_item,
#menu_id li.current_page_item li {
 display: block;
}

Replace #menu_id by your menu ID, or by .widget_pages to do this using the Pages widget.