Высота дочернего блока

.parent { 
  position: relative; 
} 
 
.child { 
  overflow: hidden; 
  position: absolute; 
  top: 0; 
  bottom: 0; 
  background: red; 
}

<div class="parent"> 
  <div class="child">child</div> 
</div>

.parent { 
  position: relative; 
  height: 50px; 
} 
 
.child { 
  overflow: hidden; 
  position: absolute; 
  top: 0; 
  bottom: 0; 
  background: red; 
}

<div class="parent"> 
  <div class="child">child</div> 
</div>