将get_pages() 为你工作？http://codex.wordpress.org/Function_Reference/get_pages

如果我正确理解了你的问题，你会想要这样的东西：

$children_of_page = get_pages(array(
    \'child_of\' => $post->ID,
    \'parent\' => $post->ID  //defining both parent and child forces the list to only include direct children.
));
$children_of_first_child = get_pages(array(
    \'child_of\' => $children_of_page[0][ID],
    \'parent\' => $children_of_page[0][ID]
));

wp_list_pages(array( //Get first level children of current page and output as list
    \'child_of\' => $post->ID,
    \'depth\' => 1,
    \'title_li\' => \'\'
));
$children = get_pages(array(
    \'child_of\' => $post->ID,
    \'parent\' => $post->ID
));
wp_list_pages(array( //output the children of the first child
    \'child_of\' => $children[0][ID],
    \'depth\' => 1,
    \'title_li\' => \'\'
));