Description:
------------
For children nodes SimpleXMLElement::xpath searches the matching xPath in parent 
node.

Workaround:
Create new instances of SimpleXMLElement for each child.
$xmlNode = new SimpleXMLElement($xmlNode->asXML());

Test script:
---------------
$xmlData = <<<XML
<foo>
    <bar>
        <a>first</a>
    </bar>
    <bar>
        <a>second</a>
    </bar>
</foo>
XML;

$xmlObj = new SimpleXMLElement($xmlData);
foreach ($xmlObj->children() as $xmlNode) {
    echo "<br>" . htmlspecialchars($xmlNode->asXML()) . "<br>";
    var_dump($xmlNode->xpath("//a"));
}

Expected result:
----------------
<bar> <a>first</a> </bar>
array(1) { [0]=> object(SimpleXMLElement)#333 (1) { [0]=> string(5) "first" } } 
<bar> <a>second</a> </bar>
array(1) { [0]=> object(SimpleXMLElement)#333 (1) { [0]=> string(6) "second" } }

Actual result:
--------------
<bar> <a>first</a> </bar>
array(2) { [0]=> object(SimpleXMLElement)#333 (1) { [0]=> string(5) "first" } 
[1]=> object(SimpleXMLElement)#335 (1) { [0]=> string(6) "second" } } 
<bar> <a>second</a> </bar>
array(2) { [0]=> object(SimpleXMLElement)#331 (1) { [0]=> string(5) "first" } 
[1]=> object(SimpleXMLElement)#333 (1) { [0]=> string(6) "second" } }