Insert Text Into A Text Area Within An Iframe?

I have a page within which is an iframe, and within that iframs is a text area. I need to be able to insert text into that text area from the mother page. I have tried to insert it with the following code but it's not working :

<iframe src ="http://iframepage.com" id="myIframe">
<p>Your browser doesn't handle iframes</p>
</iframe>
<input type="button" value="Go" onclick="document.getElementById('myIframe').contentDocument.getElementById('textArea').value = 'test successful';">

Inside the iframe is this code for the text area :

<input class="text" size="50" name="area" id="textArea" value="" maxlength="255" onkeyup="checkconditions(this.value, this.name, this.type)" type="text">

I've also tried to insert it using this script :

<script>
function insertText() {

[code]....

But nothing works.