CSS: last-child of parent

You can use .parent > *:last-child or just .parent > :last-child

An asterisk (*) is the universal selector for CSS. It matches a single
element of any type. Omitting the asterisk with simple selectors has
the same effect.

.parent > *:last-child {
  background-color: red;
}

<div class="parent">
  <p>First child</p>
  <input type="text" placeholder="Second child" />
  <div>Third child</div>
</div>