How to select an element which parent is not specified class?

I want to hide an element if its parent does not have a certain class:

HTML

<li class="current_page_parent">
    <a href="parent.html">Parent</a>
    <ul class="children">
        <li>foo</li>
        <li>bar</li>
    </ul>
</li>

jQuery

jQuery("ul.children").hide();

Currently always hides <ul class="children"> regardless of class. I would like to close it only if parent is :not an <li class="current_page_parent">.

I've tried:

jQuery("ul.children:not(:parent.current_page_ancestor)").hide();

with no luck.