SOURCE

<div class="wrapper container">
  <div class="child"></div>
  <div class="child"></div>
  <div id="child3" class="child" style="margin-top: 20px"></div>
</div>

.wrapper {
  background-color: yellow;
  width: 200px;
  height: 200px;
}

.wrapper.container {
  background-color: grey;
}

.wrapper .child {
  width: 50px;
  height: 50px;
  background-color: red;
  margin-top: 5px;
}

.wrapper > .child {
  background-color: green;
}

.wrapper .child#child3 {
  margin-left: 10px;
  margin-top: 5px;
}