Selecting from same class the first child and adding a class name

I have two classes with nested content, ex:

<div class="tab-content">
  <div class='tab-pane'></div>
  <div class='tab-pane'></div>
</div>

<div class="tab-content">
  <div class='tab-pane'></div>
  <div class='tab-pane'></div>
</div>

How do I add the class active on the first tab-pane for each tab-content?