Make Child Div Overflow Inside Parent Div?

I've looked a dozens of questions and answers and searched for hours and I cannot find any information on this specific scenario. So I have a parent div with a child div inside it.

Solution 1:

#parentDiv {
  background: red;
  height: 200px;
  overflow-y: scroll;
}

#childDiv {
  background: green;
}

<fieldset id='parentDiv'>
  <div id='childDiv'>
    <p>some text yo</p>
    <p>some text yo</p>
    <p>some text yo</p>
    <p>some text yo</p>
    <p>some text yo</p>
    <p>some text yo</p>
    <p>some text yo</p>
    <p>some text yo</p>
    <p>some text yo</p>
    <p>some text yo</p>
    <p>some text yo</p>
  </div>
</fieldset>

You need to add overflow to the parent: overflow-y: scroll;