Это должно работать:
$fC = $div.children("p:first-child");
$fC.html($fC.html().substring(0, 100));
Проверено: у меня это сработало. Мой код макета: http://pastebin.com/f737f7ce9
<!DOCTYPE html>
<html>
<head>
<title></title>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js" type="text/javascript"></script>
</head>
<body>
<div>
<p>Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum.</p>
<p>It is a long established fact that a reader will be distracted by the readable content of a page when looking at its layout. The point of using Lorem Ipsum is that it has a more-or-less normal distribution of letters, as opposed to using 'Content here, content here', making it look like readable English. Many desktop publishing packages and web page editors now use Lorem Ipsum as their default model text, and a search for 'lorem ipsum' will uncover many web sites still in their infancy. Various versions have evolved over the years, sometimes by accident, sometimes on purpose (injected humour and the like).</p>
</div>
<div>
<p>Integer ut sapien vitae orci vehicula mattis at eget lacus. Duis enim purus, sagittis nec euismod vitae, laoreet sit amet augue. Sed lacus risus, congue ac cursus et, consectetur at erat. Vestibulum lobortis tincidunt risus, in malesuada lacus ultrices et. Nulla commodo lectus pharetra purus pulvinar auctor. Duis urna felis, consequat ac facilisis id, ultrices ac tellus. Aliquam id semper dolor. Aenean eu augue augue. Vestibulum a elit turpis. Sed sed dignissim augue. Maecenas malesuada aliquet mi, sit amet facilisis quam posuere ut. Nulla luctus nulla vitae est vehicula mattis. Sed in nisi ac urna egestas condimentum. Nulla facilisi. Proin sit amet justo purus. </p>
<p>Vestibulum ut elit neque, eget blandit nulla. Mauris tortor nulla, tincidunt gravida placerat vel, iaculis nec ipsum. Donec sed quam massa, at dignissim arcu. Sed porta mattis elementum. Cum sociis natoque penatibus et magnis dis parturient montes, nascetur ridiculus mus. In hac habitasse platea dictumst. Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Curabitur vel turpis eget urna eleifend egestas sit amet ut nibh. Integer vitae varius neque. Nullam leo libero, condimentum in mollis ut, ornare et nulla. Etiam at quam quis sapien rutrum porttitor. Ut pellentesque mi vitae odio pellentesque sagittis. Vestibulum ante ipsum primis in faucibus orci luctus et ultrices posuere cubilia Curae; Phasellus a sapien nulla, a ultricies nibh. Curabitur lectus quam, rhoncus quis elementum a, congue auctor sem. </p>
</div>
<script type="text/javascript">//<!--
$(window).ready(function(){
$("div").each(function(){
var $div = $(this);
var $fC = $div.children("p:first-child");
$fC.html($fC.html().substring(0,100));
});
});
// --></script>
</body>
</html>