acf.php

<?php

/**
 * ACF Rule Match: Page Ancestor
 *
 * @author Bill Erickson
 * @see http://www.billerickson.net/acf-custom-location-rules
 *
 * @param boolean $match, whether the rule matches (true/false)
 * @param array $rule, the current rule you're matching. Includes 'param', 'operator' and 'value' parameters
 * @param array $options, data about the current edit screen (post_id, page_template...)
 * @return boolean $match
 */
function ea_acf_rule_match_page_ancestor( $match, $rule, $options ) {
	
	if ( ! $options['post_id'] || 'page' !== get_post_type( $options['post_id'] ) )
		return false;
		
	$ancestors = get_ancestors( $options['post_id'], 'page' );
	$is_ancestor = in_array( $rule['value'], $ancestors );
	
	if ( '==' == $rule['operator'] ) { 
		$match = $is_ancestor;
	
	} elseif ( '!=' == $rule['operator'] ) {
		$match = ! $is_ancestor;
	}
	
	return $match;

}
add_filter( 'acf/location/rule_match/page_ancestor', 'ea_acf_rule_match_page_ancestor', 10, 3 );