jQuery Created Nested List Based on Attribute
<ul>
    <li id="10">Parent Item 1</li>
    <li id="14">Parent Item 2</li>
</ul>
<ul>
    <li class="10">Child 1</li>
    <li class="10">Child 2</li>
    <li class="14">Child X</li>
</ul>

<ul class="parents">
    <li id="10">Parent Item 1</li>
    <li id="14">Parent Item 2</li>
</ul>
<ul class="children">
    <li class="10">Child 1</li>
    <li class="10">Child 2</li>
    <li class="14">Child X</li>
</ul>

$(document).ready(function(){
    $('ul.children li').each(function(){
        probable_parent = $('ul.parents li#' + $(this).attr('class'));
        if (probable_parent.length)
        {
            if (!probable_parent.find('ul').length) probable_parent.append('<ul/>');
            $(this).detach().appendTo(probable_parent.find('ul'));
        }
    });
});

<ul class="parents">
    <li data-itemid="10">Parent Item 1</li>
    <li data-itemid="14">Parent Item 2</li>
</ul>
<ul class="children">
    <li data-itemid="10">Child 1</li>
    <li data-itemid="10">Child 2</li>
    <li data-itemid="14">Child X</li>
</ul>