1. 先取出第一層分類，請參考下列文章
WordPress 只取第一層分類
2. 用 get_term_children(); 所有子分類id
/*
取所有仔分類ID
https://v123.tw
*/
foreach($categories as $cat){
echo 'name='.$cat->name.'<br/>';
$term_children = get_term_children( $cat->term_id , 'category' );
if($term_children){
echo 'child_id='.implode(",",$term_children).'<br/>';
}
echo '------------<br/><br/>';
}3. 結果：
name=aaaaaaaaaaaa ------------ name=bbbbbbbbbbbb child_id=36,37 ------------ name=cccccccccccc ------------ name=dddddddddddd child_id=32,33,34,38 ------------