Edit in JSFiddle

$(document).ready(function(){
    var str = 'No of children: '
    $('p').html(str + $('#tblData').children().length);
});

<table id='tblData' border='1'>
<tr>
    <td> Cell 1 </td>
</tr>
<tr>
    <td> Cell 2 </td>
</tr>
<tr>
    <td> Cell 3 </td>
</tr>
</table>
<br/>
<p></p>