Redirect Product Child Category to full URL that includes the Parent Category

Question

I modified the URL structure for Product child categories so that it shows like: domain.com/product_parent_cat/product_child_cat/ which works fine.

The problem is that the child category is still accesible without the parent, which can cause a lot of SEO problems.

domain.com/product_child_cat/ this loads the exact same page. I would like to avoid having to add multiple redirects. Has anyone faced an issue like this before?

This is the code I'm using to modify the URL structure

    global $wpdb;
    if( ! empty( $vars['pagename'] ) || ! empty( $vars['category_name'] ) || ! empty( $vars['name'] ) || ! empty( $vars['attachment'] ) ) {
        $slug = ! empty( $vars['pagename'] ) ? $vars['pagename'] : ( ! empty( $vars['name'] ) ? $vars['name'] : ( !empty( $vars['category_name'] ) ? $vars['category_name'] : $vars['attachment'] ) );
        $exists = $wpdb->get_var( $wpdb->prepare( "SELECT t.term_id FROM $wpdb->terms t LEFT JOIN $wpdb->term_taxonomy tt ON tt.term_id = t.term_id WHERE tt.taxonomy = 'product_cat' AND t.slug = %s" ,array( $slug )));
        if( $exists ){
            $old_vars = $vars;
            $vars = array('product_cat' => $slug );
            if ( !empty( $old_vars['paged'] ) || !empty( $old_vars['page'] ) )
                $vars['paged'] = ! empty( $old_vars['paged'] ) ? $old_vars['paged'] : $old_vars['page'];
            if ( !empty( $old_vars['orderby'] ) )
                    $vars['orderby'] = $old_vars['orderby'];
                if ( !empty( $old_vars['order'] ) )
                    $vars['order'] = $old_vars['order'];
        }
    }
    return $vars;
});