<div class='parent'>
  <div class='child'></div>
  <div class='child2'></div>
</div>

.parent {
  padding: 10px;
  display: flex;
  justify-content: space-between;
  align-items: stretch;
  background: red;
}

.child {
  background: white;
  width: 50px;
  height: 100px;
}

.child2 {
  width: 50px;
  background: black;
}