Filter results wordpress by ancestor page

Question

I have this kind of structure

- Page A
---- Page A1
---- Page A2
---- Page Axx
- Page B
---- Page B1
---- Page B2
---- Page Bxx
- Page C
...

Ok you get it :)

Now I'd like to filter the search results of page by ancestor.

Result page should be :

Page ancestor B
- PAge B2
- Page B5

Page ancestor D
- Page D4
- Page D6

Do you think it's possible ?

add_action( "pre_get_posts", "custom_search_order" );

function custom_search_order( $query )
{
    if ( $query -> is_search() )
    {
        $query -> set( "meta_key", "service" ); // Your custom field ID.
        $query -> set( "orderby", "meta_value" ); // Or "meta_value_num".
        $query -> set( "order", "ASC" ); // Or "DESC".
    }
}

This code isn't working fine at this time.