Query child’s child categories

Ok then, I actually was getting the subcats with the wrong method.

in $parent_cat = get_cat_ID( $cat_name ); $cat_name wasn’t a name but un slug. First mistake.

So I corrected with $parent_cat = get_category_by_slug( $cat_slug );

The final code is :

// Get the parent cat thanks to the page's cat slug
$parent_cat = get_category_by_slug( $cat[1] );
$args = array(
    'type'      => 'post',
    'parent'    => $parent_cat->term_id,
    'order'     => 'DESC'
// child cats
$categories = get_categories( $args );
// loop on the child cats to get the sub cats object
foreach ($categories as $key => $value) {
    return $value;

That’s it.