add children and grandchildren to array using mysql_query WordPress

I'm trying to accomplish the following to run queries on a WordPress database:

1. use a mysql PDO prepare function to retrieve child posts of selected parents
2. use another PDO prepare function to retrieve grandchild posts
3. incorporate all of these into a single array (same key)

I've gotten as far as displaying the children, but having trouble adding grandchildren into the array. Here's the code:

$id = $_REQUEST['selected'];
$a = implode(',', array_map('intval', $id));

$hostname = 'localhost';
$username = 'victoria_wp_site';
$password = 'PASSWORD';

$db = new PDO("mysql:dbname=victoria_wp_site;host=$hostname", $username, $password);

$qry = $db->prepare("SELECT id,post_title 
          FROM `wp_posts` 
         WHERE `post_parent` IN ($a) and post_name not like '%revision%' ");
$qry->execute();

$i = 0;
while ($data = $qry->fetch()) {
    $result[$i] = $data;
    $i++;
}
        
$b = implode(',', array_column($result, 'id'));

$qry_grandchild = $db->prepare("SELECT id, post_title 
          FROM `wp_posts` 
         WHERE `post_parent` IN ($b) AND post_name not like '%revision%' ");
$qry_grandchild->execute();      

     
while ($data = $qry_grandchild->fetch()) {
    $result[$i] = $data;
    $i++;
}       

echo json_encode($result);

Not sure what I'm doing wrong.

The problem was with the format of the array used to select the grandchildren. I solved this by adding the implode function to the array before using it to select grandchildren.

$b = implode(',', array_column($result, 'id'));

$qry_grandchild = $db->prepare("SELECT id, post_title 
          FROM `wp_posts` 
         WHERE `post_parent` IN ($b) AND post_name not like '%revision%' ");
$qry_grandchild->execute();