get the category or categories of the single post with get_the_category()
and loop through them, calling the child category lists if available.
<ul>
<?php
if( is_single() ) {
$post_cats = get_the_category();
foreach( $post_cats as $post_cat ) {
wp_list_categories( array(
'child_of' => $post_cat->term_id,
'title_li' => 0,
'show_option_none' => ''
) );
}
}
?>
</ul>