$parentId = get_category_by_slug('スラッグ')->cat_ID;
/子のカテゴリID一覧取得/
$childs = get_term_children($parentId, 'category');
$chidList = '';
foreach ($childs as $chId){
$chidList = $chidList .',' .$chId;
}
$parentId = get_category_by_slug('スラッグ')->cat_ID;
/子のカテゴリID一覧取得/
$childs = get_term_children($parentId, 'category');
$chidList = '';
foreach ($childs as $chId){
$chidList = $chidList .',' .$chId;
}