SOURCE

<div class="line-box">
        <div class="line"></div>
        <div class="child-box1">
        </div>
        <div class="child-box2">
        </div>
        <sup>sup test</sup>
    </div>

.line-box {
        position: relative;
        padding-left: 100px;
        border: 1px solid black;
    }
    .line-box::after {
        content: 'x';
    }
    .child-box1, .child-box2 {
        display: inline-block;
        width: 100px;
        height: 100px;
        border: 1px solid red;
    }
    .child-box1 {
        vertical-align: super;
    }
    .line {
        position: absolute;
        top: 101px;
        height: 1px;
        width: 300px;
        background: black;
    }