Function to return true if the current page matches or is a child of a given slug. Call it like is_page_or_child('about');

wp-is-page-or-child

function get_ID_by_slug($page_slug) {
    $page = get_page_by_path($page_slug);
    if ($page) {
        return $page->ID;
    } else {
        return null;
    }
}

function is_page_or_child($slug){

  $ispage_ID = get_ID_by_slug($slug);
	if (is_page($ispage_ID) || in_array($ispage_ID, get_ancestors(get_the_ID(), 'page')) ) {
		return true;
	}
}