Current build version of WebGrid doesn’t provide style customization over child table yet. So, I’m afraid that currently it is not possible to put a border along the bottom of the child table.

The empty column at the left is the column that is used as the row selector of the child table. Please try to set the RowHeaders property in the LayoutSettings of WebGrid to “No” in order to losing the empty columns at the left.

...
<LayoutSettings Hierarchical="True" AllowFilter="Yes" AllowExport="Yes" RowHeaders="No">
</LayoutSettings>
...

WebGrid has HidePadColumn property which hides the pad column (a small section of the column headers above the vertical scrollbar). Please try to disable this property in window on load JavaScript function in order to losing the emptly columns at the right.

Hope this helps.