Issue on Listing Sub Product Categories In Woocommerce

To get children with get_terms() you have two options. You can either use the argument parent, which will only get direct children, so only one level down; or you use child_of, which gets all descendants, meaning until last level down. Both parent and child_of need an integer as value, which would in both cases be the parents ID.

Basic code example:

$taxonomies = array(
    'product_cat'
);
$args = array(
    // in case you haven't assigned the terms to products
    'hide_empty' => 0,
    // replace with the parent ID you need
    'parent'     => 10
);
$product_categories = get_terms(
    $taxonomies,
    $args
);