if ( ! is_admin() )
    add_filter( 'wp_get_nav_menu_items', 'viper_add_child_pages_to_menu', 10, 3 );

function viper_add_child_pages_to_menu( $items, $menu, $args ) {
    $menu_order = count( $items ) + 1000;

    foreach ( $items as $item ) {
        // Skip menu items that aren't top level or aren't post_type=page
        if ( 0 != $item->menu_item_parent || 'page' != $item->object )
            continue;

        // Get all decendent pages
        $children = get_pages( array(
            'child_of' => $item->object_id,
            'sort_column' => 'menu_order',
        ) );

        // Add each child page to the menu
        foreach ( $children as $child ) {
            $child = wp_setup_nav_menu_item( $child );
            $child->db_id = $child->ID;

            // Set the parent menu item.
            // If the current page is a child of the current menu item,
            // then use that menu item's ID otherwise use the real ID
            // because it's a menu item that this plugin added itself.
            if ( $child->post_parent == $item->object_id )
                $child->menu_item_parent = $item->ID;
            else
                $child->menu_item_parent = $child->post_parent;

            // The menu_order has to be unique, so make up new ones
            // The items are already sorted anyway due to the get_posts()
            $menu_order++;
            $child->menu_order = $menu_order;

            $items[] = $child;
        }
    }

    return $items;
}