<?php
if ( is_category() ) {
$current_cat = get_query_var('cat');
wp_list_categories('depth=1&hide_empty=0&title_li=&show_count=1&child_of='.$current_cat);
}
?>
$category = get_queried_object();
$child_categories = get_term_children( $category->term_id, 'category');
if( $category instanceof WP_Term ) {
$child_categories = get_term_children( $category->term_id, 'category');
}