/ Published in: PHP
Usually use in a template with `in_category()`, like `if ( in_category( 3 ) || in_subcategory( 3 ) ) { ...`. Tested in WP 2.8.4.
URL: http://codex.wordpress.org/Function_Reference/get_term_children
URL: http://codex.wordpress.org/Function_Reference/get_term_children