Elliot finally got it going for me through personal support. If anyone is still interested, here is the code:
/* ACF: limit child-page selection to current page’s children */
function childpages_query( $args, $field, $post_ID )
{
$args['post_parent'] = $post_ID;
return $args;
}
add_filter('acf/fields/relationship/query/name=pages_childpages', 'childpages_query', 10, 3);
while pages_childpages is the relationship field. It finally works in my case ;).
