It might not be possible to completely remove the transparency from a translucent parent element. In this scenario, you can opt to make the parent element's background translucent while maintaining the opacity levels:
body {background: url("https://i.imgur.com/vifaloim.jpg"); margin: 25px;}
.parent {background: rgba(255,255,255,0.75); margin: 10px; border-radius: 10px; padding: 10px;}
.child {margin: 10px; background: #fff; border-radius: 10px; padding: 10px; min-height: 100px;}
<div class="parent">
<div class="child"></div>
</div>