RecursiveArrayIterator::getChildrens returns a copy of the children, not a reference:
<?php
$stack = array ("some" => "value",
                array ("subsome" => "subvalue", array ("subsubsome" => "subsubvalue")),
                "some1" => "value1");

$object = new RecursiveArrayIterator ($stack);
$object->next ();
$second_object = &$object->getChildren ();

$second_object->next ();

$third_object = &$second_object->getChildren ();

$third_object->offsetSet ("subsubsome", "subsubdiferent");

var_dump ($object);
var_dump ($second_object);
var_dump ($third_object);
?>

returns: 

object(RecursiveArrayIterator)#1 (1) {
  ["storage":"ArrayIterator":private]=>
  array(3) {
    ["some"]=>
    string(5) "value"
    [0]=>
    array(2) {
      ["subsome"]=>
      string(8) "subvalue"
      [0]=>
      array(1) {
        ["subsubsome"]=>
        string(11) "subsubvalue" <--- expected to be changed
      }
    }
    ["some1"]=>
    string(6) "value1"
  }
}
object(RecursiveArrayIterator)#2 (1) {
  ["storage":"ArrayIterator":private]=>
  array(2) {
    ["subsome"]=>
    string(8) "subvalue"
    [0]=>
    array(1) {
      ["subsubsome"]=>
      string(11) "subsubvalue" <--- expected to be changed
    }
  }
}
object(RecursiveArrayIterator)#3 (1) {
  ["storage":"ArrayIterator":private]=>
  array(1) {
    ["subsubsome"]=>
    string(14) "subsubdiferent"
  }
}