Conditional for is page and page’s Children

In the functions file…

function is_tree($pid) {      // $pid = The ID of the page we're looking for pages underneath
	global $post;         // load details about this page
               return true;   // we're at the page or at a sub page
               return false;  // we're elsewhere

In the theme…

if (is_tree(8)) { //the number being the page ID
   // do stuff