The get_term_children() WordPress PHP function merges all term children into a single array of their IDs.
Usage
get_term_children($term_id, $taxonomy_name)
Input:
$term_id: 10, $taxonomy_name: ‘products’
Output:
Array of children term IDs
Parameters
$term_id(int): Required ID of term to get children.$taxonomy_name(string): Required taxonomy name.
More information
See WordPress Developer Resources: get_term_children
Note
Categories and Tags are the two pre-defined taxonomies in WordPress. The Taxonomy Name, the second required parameter $taxonomy, is ‘category’ for Categories and ‘post_tag’ for Tags. If replacing the deprecated function get_category_children(), which returns a string, note that get_term_children() returns an array of category IDs if the second parameter $taxonomy is ‘category’.
Examples
Displaying child terms in an unordered list
This example gets an array of children taxonomies and displays them as links in an unordered list.
$term_id = 10;
$taxonomy_name = 'products';
$termchildren = get_term_children($term_id, $taxonomy_name);
echo '<ul>';
foreach ($termchildren as $child) {
$term = get_term_by('id', $child, $taxonomy_name);
echo '<li><a href="' . get_term_link($child, $taxonomy_name) . '">' . $term->name . '</a></li>';
}
echo '</ul>';