I wonder if this will work for you:
function search_filter( $query )
{
if( $query->is_search AND $query->is_main_query() )
{
// $query->set( 'post_parent__not_in', array( 0 ) ); // We comment this out here
$query->set( 'post_type', array( 'post', 'page' ) );
}
return $query;
}
add_action( 'pre_get_posts', 'search_filter' );
and
function wpse_120638( $where, $query )
{
global $wpdb;
if( $query->is_search AND $query->is_main_query() )
{
$from = "AND $wpdb->posts.post_type IN ('post', 'page')";
$to = "AND ( ( $wpdb->posts.post_parent NOT IN (0)
AND $wpdb->posts.post_type IN ('page') )
OR $wpdb->posts.post_type IN ('post') ) ";
$where = str_replace( $from, $to, $where );
}
return $where;
}
add_filter( 'posts_where', 'wpse_120638', 10, 2 );