how to make child in flex go full width?

.dashboard-body{
  text-align:center;
  width: 300px;
  margin: 0 auto;
}
.btn-group{
  display:flex;
  flex-direction:row;
  justify-content: space-between;
}
.btn-group a{
  flex:0 0 auto;
}

<div class="dashboard-body">
  <p>You don't have any sales data.</p>
  <p>Please add a new book or upload CSVs.</p>

  <div class="btn-group">
    <a href="#">
      <input class="add-new-book-btn" type="submit" value="Add New Book">
    </a>
    <a href="#">
      <input class="add-new-book-btn" type="submit" value="Upload CSV">
    </a>
  </div>
</div>