How to amend time format of comments, using child-theme?

You have to return the formatted date string, the following will work:

// define the get_comment_time callback 
function filter_get_comment_time( $date, $d, $gmt, $translate, $comment ) { 
  $d = "g:i:s";
  $date = mysql2date($d, $date, $translate);
  return $date;
}; 

// add the filter 
add_filter( 'get_comment_time', 'filter_get_comment_time', 10, 5);