.flex-child
{   width: 6px;
    float: left;
    border: 1px solid yellow;
    background-color: yellow;
    height: 100%;
    margin: 1px;
}
.center {
    height: 100%; 
    overflow: auto;
    margin: auto;
    width: 60%;
    height: 100%;
    padding-top: 5%;
  }