How to query for a page, get data, then query for child pages of that page

I don’t understand the distinction you are making between ‘sublevel’ query and ‘toplevel’ query, but this is how I pull children for a page.

$children = get_children(
    array(
        'post_parent'   => $servid,
        'post_type'     => 'page',
        'post_status'   => 'publish',
        'orderby'       => 'menu_order',
        'order'         => 'ASC'
)); ?>

Or…

$page_children = get_pages(
 array(
  'child_of'    => $post->ID,
  'post_type'   => 'page',
  'sort_column' => 'menu_order',
  'order'       => 'ASC'
 ));

In both cases I am sending the parent post’s ID as the first parameter.

http://codex.wordpress.org/Function_Reference/get_children

http://codex.wordpress.org/Function_Reference/get_pages