<!DOCTYPE html>
<style>
.flexContainer {
  display: flex;
  flex-direction: column;
  position: relative;
  height: 100px;
}
.firstChild {
  background-color: yellow;
  height: 50px;
}
.secondChild {
  height: 50%;
  background-color: green;
}
.percentHeightDescendant {
  height: 100%;
  background-color: green
}
</style>
<div class=flexContainer>
  <div class=firstChild><div class=percentHeightDescendant></div></div>
  <div class=secondChild></div>
</div>