Sub-category which shows posts list of parent category

Don’t ask me why but I have this need on my site:

I have the following category structure which is also reflected in the menu:

cat 1 (parent)
cat 2 (sub)
cat 3 (sub)
cat 4 (sub)
cat 5 (sub)

Every link of the menu redirect to the relative page of the category which show the posts list.

I need ‘cat 2’ to show not only posts of ‘cat 2’ but ALL the posts of the sub-categories of ‘cat 1’ (therefore cat 2,3,4 and 5)

I made the query using tag is_category but I get errors like “syntax error, unexpected ‘)'” and so on. This is the query I used:

<?php if (is_category('magazine')) { query_posts('cat=61'); ?>
<?php } elseif ()) { ?>
<?php global $query_string; query_posts($query_string); ?>
<?php if (have_posts()) : while (have_posts()) : the_post(); ?>
<?php } else { ?><?php } ?>

Anyone has an idea on how to write a query for my needs?
(A sub-category which shows the posts list of ALL the posts belonging to the parent category)

Thanks in advance…