Modifications to functions.php for Twenty Ten child theme not showing

I am also trying to modify my functions.php in a child theme. Ultimately I want to exclude specific categories from my blog home page. I am using this code, but it doesn’t seem to do anything.

<?php
function exclude_category($query) {
if ( $query->is_home ) {
$query->set('cat', '-10 -11 -12');
}
return $query;
}
add_filter('pre_get_posts', 'exclude_category');
?>