WP: is_tree() function to check if the current page is the page or a sub page of the page ID specified. Can be used as conditional tag with Widget Logic. Source: http://codex.wordpress.org/Conditional_Tags#A_PAGE_Page

is_tree.php

<?php

// This function will return true if we are looking at the page in question or one of its sub pages
function is_tree( $pid ) {      // $pid = The ID of the page we're looking for pages underneath
    global $post;               // load details about this page

	if ( is_page($pid) )
		return TRUE;            // we're at the page or at a sub page

	$anc = get_post_ancestors( $post->ID );
	foreach ( $anc as $ancestor ) {
		if( is_page() && $ancestor == $pid ) {
			return TRUE;
		}
	}

	return FALSE;  // we aren't at the page, and the page is not an ancestor
}