How to load children of a Taxonomy Term in Drupal 10

call the taxonomy class.

use Drupal\taxonomy\Entity\Term;

To get the child terms using parent term id,

$parent_id = 'Parent Term Id';

$vocabulary = 'Taxonomy name';

function getChildren(string $parent_id): array {

  $child_terms = \Drupal::entityTypeManager->getStorage('taxonomy_term')->loadTree(

    $vocabulary,

    $parent_id,

    NULL,

    FALSE 

  );

   $children = []; 

    if(empty($child_terms)) {

    //This is a parent tid with no child terms.

    return $parent_id;

  } 

  foreach ($child_terms as $term) {  

    $children[] = $term->tid; 

  } 

  return $children;

}