<div class="container">
  <div>first child</div>
  <div>second child</div>
</div>

* {
  padding: 0;
  margin: 0;
  box-sizing: border-box;
}

div {
  border: 1px solid green;
}

.container {
  display: flex;
  flex-flow: column wrap;
  justify-content: center;
  height: 100vh;
}