<div class="Father">

      <div class="LeftChildren">我是左侧</div>

      <div class="RightChildren">我是右侧</div>

    </div>

<style  rel="stylesheet/scss" lang="scss">

  .Father {

    border: 1px solid red;

    height: 100px;

  }

  .LeftChildren {

    width: 50px;

    border: 1px solid blue;

    float: left;

  }

  .RightChildren {

    width: calc(100% - 50px);

    border: 1px solid green;

    float: right;

  }

}

</style>

<div class="Father">

      <div class="LeftChildren">我是左侧</div>

      <div class="RightChildren">我是右侧</div>

    </div>

<style  rel="stylesheet/scss" lang="scss">

  .Father {

    border: 1px solid red;

    height: 100px;

    display: flex;

  }

  .LeftChildren {

    width: 50px;

    border: 1px solid blue;

  }

  .RightChildren {

    border: 1px solid green;

    flex: 1;

  }

</style>