Seems your issue is loading the XML, not the json_encoding.
Your print_r for the $simple object has the following for the Consultant (an empty object)
[consultant] => SimpleXMLElement Object ( )
I found this in the comments on php.net
If you want CDATA in your object you should use LIBXML_NOCDATA
$xml = simplexml_load_file($file_xml, 'SimpleXMLElement',LIBXML_NOCDATA);