When passing a selector to $('#div2').children() in the form of [parent] > [child]...

• if the :first selector is added to [parent],
• and there are no immediate children that match [child],
• but there is an immediate child that matches [parent],
• and [parent] has an immediate child that matches [child]

... jQuery/Sizzle will return [parent].

http://jsfiddle.net/F62c3/

<div id='div2'>
    <ul id='ul2'>
        <li>4</li>
        <li>5</li>
        <li>6</li>
    </ul>
</div>

// Using "ul > li" selector returns an empty set (as it should)
$('#div2').children('ul > li'); 

// Using "ul:first > span" selector returns an empty set (as it should)
$('#div2').children('ul:first > span');

// Using "ul:first > li" selector returns the UL even though we're targeting LI
$('#div2').children('ul:first > li');

NOTE: The same applies to :last and :eq(0), but not :first/last/nth-child. I haven't tested beyond this.