I've not seen your code, but I mocked up a similar scenario.
HTML
<div id="main">
<div id="first">
<table>
<tr>
<td>A</td>
<td>B</td>
</tr>
</table>
</div>
<div id="second">
<table>
<tr>
<td>A</td>
<td>B</td>
</tr>
<tr>
<td>A</td>
<td>B</td>
</tr>
<tr>
<td>A</td>
<td>B</td>
</tr>
<tr>
<td>A</td>
<td>B</td>
</tr>
</table>
</div>
</div>
As you can see, the height of the second table is "dynamic", and it could be longer than the first table, doesnt matter.
The CSS
#main {
width:500px;
overflow:hidden;
}
#first, #second {
padding-bottom: 1000px;
margin-bottom: -1000px;
float: left;
}
#first {
float:left;
width:100px;
overflow:auto;
}
#second {
width:400px;
float:left;
}
Thus far, what you have is the #first parent to follow the height of the #second. Reference
Fiddle
So what now? The #first follows the height of the #second, but the #first_child does not follow the height of #first. However, HTML tables does not follow parents div's heights. Reference
Answer: Javascripts.
You first want to detect the height of the #second, and then auto adjust the height of the #first_child to follow the height of the #second.
var second_height = $("#second").height();
var table_height = second_height;
$("#first_child").height(table_height);
Solution
Hope this is what you're looking for.