How to call stream_publish on a child Facebook app using the Facebook PHP API

$facebook = new Facebook($childApiKey, $sessionSecret);
$facebook->api_client->session_key = $sessionKey;
$facebook->api_client->use_session_secret($sessionSecret);
$facebook->api_client->stream_publish('Hello!');

where $sessionKey and $sessionSecret appear in $_COOKIE['[apiKey]_session_key'] and $_COOKIE['[apiKey]_ss'].