How to put the child class to the parent?

How to put the child class to the parent?

Hello again, how i can put the child class to the parent?

For example, i have this:
  1. <div class="parent">
  2. <div class="child-one">Content of child one</div>
  3. </div>
  4. <div class="parent">
  5. <div class="child-two">Content of child two</div>
  6. </div>
  7. <div class="parent">
  8. <div class="child-three">Content of child three</div>
  9. </div>
And I want add to the parent div the class of his child, thus:
  1. <div class="parent child-one">
  2. <div class="child-one">Content of child one</div>
  3. </div>
  4. <div class="parent child-two">
  5. <div class="child-two">Content of child two</div>
  6. </div>
  7. <div class="parent child-three">
  8. <div class="child-three">Content of child three</div>
  9. </div>
Thanks in advance. :)