As far as i known clicking the children elements won't work because it is nested inside a button. A button is not container for child elements - only for text.

If you try like below it always tell that you clicked the button.

<button type=button onclick=alert('You clicked the button!');>

   inside a button <span onClick=alert('clicked span');>**Not working**</span>

</button>

So, my suggestion is to use another span or a div

<div class=button_replace>

  inside a div <span onClick=alert('clicked span');>**working**</span>

</div>

Hope it helps...

Note: Your code will work in chrome but not in firefox and my answer is alternative solution for making work in firefox