Search anything from a Child and Sub-Pages of their Parent page

I am trying to search and want to get results only from the Child - Sub Pages of their parent page. In the following code, I just get results from child pages, not subpages. Please help

Understand the Root:

Parent Page
 child 1
  sub child a
  sub child b
 child 2
  sub child a
  sub child b
 child 3
  sub child a
  sub child b

Note: (When I search anything Results comes only from their child and not from subpages)

Function.php

?PHP
function searchquery($query) {
    if ($query-is_search) {
        $query-set('post_parent', 16);
    }
    return $query;
}
add_filter('pre_get_posts','searchquery');
?

html code

form id=search_form method=get class=search_form action=?php echo get_site_url(); ?

input type=text value=?php echo searchquery($query); ? name=s id=s placeholder=Course Name /

input type=hidden value=post name=page / 

button type=submit value=Submit search course i class=fa fa-search/i /button 
/form