php - Unable to send an associative array in JSON format in Zend to client

In one of my actions in a controller, I'm using the json view helper to send back a response to an ajax request. On the client side I alert the data that is passed to the success callback function. It works fine as long as the response is a number or an array with default keys. Once I try to send an associative array, it alerts with [object Object]. Server code:

$childArray = array('key'=>'value');
$this->_helper->json($childArray);
Copy code

javascript:

function displayChildren(data){
    alert(data);
}
...    
$.ajax({
        url: "/po/add", dataType: "json",
    data: {format: "json"}, success: displayChildren
});
Copy code

I have no idea what am I doing wrong here, so any help would be appreciated...