Edit in JSFiddle

.header > .child{
  color: blue;
}

.header  .child2{
  color: green;
}
<div class="header">
 <h1>
 welcome dear
 </h1>
 <div class="child">
 <h1>
  This is child
 </h1>
 </div>
 <div class="child2">
 <h1>
  This is child
 </h1>
 </div>
</div>