Position

<div id="outer-container">
  <div id="container">
    <div id="child-element-1">
      Child 1
    </div>
    <div id="child-element-2">
      Child 2
    </div>
    <div id="child-element-3">
      Child 3
    </div>
  </div>
</div>

body {
  background-color: steelblue;
}
div {
  border: 2px solid white;
  color: white;
  font-size: 1.2rem;
}
#outer-container {
  position: relative;
  width: 90vw;
  height: 90vh;
}
#container {
  width: 60vw;
  height: 60vh;
  margin: 30px;
}
#child-element-1 {
  position: absolute;
  top: 10px;
  right: 10px;
}
#child-element-2, #child-element-3 {
  width: 40vw;
  margin: 10px;
}