Problem with custom page template

Problem: the page template I have created doesn’t appear anywhere.

I go to page edit and under ‘Page Attributes’>’Templates’ there are only the ‘default’, ‘showcase’ and ‘sidebar’ templates.

I am using the twentyeleven theme, wordpress version 3.2.1

To create the custom template I put this at the top of a php file and put it in the themes folder.

wp-content/themes/twentyeleven/xxx.php

<?php
/*
Template Name: xxx
*/
?>

Can anyone see where I might have gone wrong?

Another question is – what makes a php file recognised as a page template? The existing page.php doesn’t have that stuff at the top, so I am confused.

By the way I am also using a child theme. Would that complicate things?? Like where to put the custom template file?

Thanks