<div id="main">
  <div class="child">元素1</div>
 ​ ​<div class="child">元素2</div>
  ​<div class="child">元素3</div>
 ​ <div class="sibling">元素4</div>
 ​ <div class="child">元素5</div>
</div>

<script>
$(document).ready(function(){
  //所有.child添加样式
  $('#main > .child').css('color', 'blue');
  
  //nth-child(2)添加样式
  $('#main > .child:nth-child(2)').css('font-weight', '500');
});
</script>