SOURCE

<div class="line-box">
         <!-- 横线，测试用，用来比对两个子元素对齐的位置 -->
              <div class="line"></div>
        <div class="child-box1"></div>
        <div class="child-box2"></div>
    </div>

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