I wrote a little function a while back to Check if a Page is a Child of Another Page in WordPress called is_child(). It used the page ID to return a true or false value dependent on if the current page was a child of the parent page specified. I have recently made a little change so that the function could now accept either a page ID or slug.

Here it is again with the changes:

function is_child($page_id_or_slug) { 
	global $post; 
	if(!is_int($page_id_or_slug)) {
		$page = get_page_by_path($page_id_or_slug);
		$page_id_or_slug = $page->ID;
	if(is_page() && $post->post_parent == $page_id_or_slug ) {
       		return true;
	} else { 
       		return false; 

The modification was inspired by Helpful Page Functions for WordPress.