<div class="outside" style="float:left; margin-right:10px">
  <div class="parent">
      <div class="child">
          Align to the bottom 1
      </div>
      <div class="child">
          Align to the bottom 2
      </div>
      <div class="child">
          Align to the bottom 3
      </div>

  </div>
</div>

<div class="outside">
  <div class="parent">
      <div class="child">
          Align to the bottom 1
      </div>
      <div class="child">
          Align to the bottom 2
      </div>
      <div class="child">
          Align to the bottom 3
      </div>
      <div class="child">
          Align to the bottom 4
      </div>
      <div class="child">
          Align to the bottom 5
      </div>
      <div class="child">
          Align to the bottom 6
      </div>
  </div>
</div>

.outside
{
    height:200px;
    width: 200px;
    border: 4px solid green;
    overflow-y:auto;
}

.parent
{
    height: 100%;
    width: 200px;
    display: flex;
    flex-direction: column;
    justify-content:flex-end;
    overflow-y:auto;
}

.child
{
    height: 40px;
    width: 100%;
    background: #f00;
    flex: 0 0 auto;
}
.child:nth-child(odd)
{
  background: red;
}

.child:nth-child(even)
{
  background: blue;
}