An extended version of WordPress is_tree and is_page conditionals for finding if a page is child or grandchild of another page by it's slug

is_tree_extended.php

/**
 * is_tree extended function.
 * 
 * Extends the typical is_tree function -- as found in WordPress Theme Handbook ( https://developer.wordpress.org/themes/basics/conditional-tags/ ) and CSS Tricks ( https://css-tricks.com/snippets/wordpress/if-page-is-parent-or-child/ ) -- and allows it to find if a page is child or grandchild of another page by it's slug
 * 
 * @access public
 * @author Celso Bessa <celso.bessa@2aces.com.br>
 * @see https://www.celsobessa.com.br/2017/07/17/is-tree-conditional-extended/
 * @param mixed $page
 * @param bool $use_slug if set to true, (default: false)
 * @return bool true if a page is child or grandchild of another page
 */
function is_tree( $page_id, $use_slug = false ) {
	
	if ( $use_slug === true && !is_string( $page_id ) ) {
		return false;
	} else if ( $use_slug !== true && !is_int( $page_id ) ) {
		return false;
	}
	
	if ( $use_slug === true ) {
		$page_data = get_page_by_path( $page_id );
		if ( null === $page_data ) {
			return false;
		}
		$page_id = $page_data->ID;
	}
	
	global $post;         // load details about this page
	if( is_page( $page_id ) ) {
		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 == $page_id ) {
            return true;
        }
    }
		
	return false;  // we're elsewhere

};