adds child pages as menu items to top level menu items

add_filter( 'wp_get_nav_menu_items', 'add_child_pages', 10, 3 );

function add_child_pages( $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 children pages
        $children = get_posts( array(
            'supress_filters' => false, // Caching, etc.
            'post_parent' => $item->object_id,
            'post_type' => 'page',
            'orderby' => 'menu_order',
            'nopaging' => true,
        ) );

        // Add each child page as a child menu item to the current menu item
        // We'll worry about proper heirarchy (grandchildren) later -- this is a test
        foreach ( $children as $child ) {
            $child = wp_setup_nav_menu_item( $child );

            $child->menu_item_parent = $item->db_id; // Child of the current menu item

            // 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;
        }
    }

    //var_dump( $items );

    return $items;
}