Javascript: Get mouse position relative to parent element

Is there any way to get mouse position relative to it's parent element?

Let's say I have a structure:

<div id="parent">
    <span class="dot"></span>
</div>

When I bring my mouse over span element I need to get its position relative to its parent element (<div id="parent">). PageX/ClientX give me position relative to page/client area, so it's not working for me.