Question

I need to have a table where each row can have a child table or other content below that row.

Is there any valid way to do this?

Answer 1

Add another <tr> after the current one with a colspan which spans all columns and then put another <td> with a <table> therein.

<tr>
    <td>col1</td>
    <td>col2</td>
    <td>col3</td>
</tr>
<tr>
    <td colspan="3">
        <table>...</table>
    </td>
</tr>
<tr>
    <td>col1</td>
    <td>col2</td>
    <td>col3</td>
</tr>

The new table will then appear between the rows.