table,th,td {
  border: 1px solid black;
  border-collapse: collapse;
}

th,td {
  padding: 5px;
}


.parent-table > tbody > tr > td:first-child {
/*                    ^^   ^^     */
/* Means 'first "td" child of first order "tr" children of ".parent-table" */
  width: 18%;
  background: red;
}

<table class="parent-table">
  <tr>
    <td>
      <label>Parent first td</label>
    </td>
    <td>
      <table>
        <tr>
          <td>Child first td</td>
          <td>Child second td</td>
        </tr>
      </table>
    </td>
  </tr>
</table>

table, th, td {
  border: 1px solid black;
  border-collapse: collapse;
}
th, td {
  padding: 5px;
}

td:first-child {
  width: 18%;
  color: red;
}

<table>
  <tr>
    <td>
     <label>Parent first td</label>
    </td>
    <td>
      <table>
        <tr>
          <td>Child first td</td>
          <td>Child second td</td>
        </tr>
      </table>
    </td>
  </tr>
</table>