one category template for multiple categories

First paste this code in your theme’s functions.php

function post_is_in_descendant_category( $cats, $_post = null ){
    foreach ( (array) $cats as $cat ) {
        // get_term_children() accepts integer ID only
        $descendants = get_term_children( (int) $cat, 'category');
        if ( $descendants && in_category( $descendants, $_post ) )
            return true;
    }
    return false;
}

then on your category.php or archive.php (depends on your theme) at the very top add

<?php if (post_is_in_descendant_category(33)){
 include (TEMPLATEPATH . '/category-events.php'); 
 exit;
} ?>

and change 33 to the ID of the ‘events’ category.

What it does is check if the current category is a child category of events and if so it uses the right theme file.