<div class="checkboxes">
  <label><input id="topList" name="list1" type="checkbox">Main Label</label>
  <label><input id="list1" name="list2" type="checkbox">First Sub Label</label>
  <label><input id="list2" name="list3" type="checkbox">Second Sub Label</label>
  <label><input id="list3" name="list4" type="checkbox">Third Sub Label</label>
</div>

.checkboxes label {
  display: block;
  margin-left: 20px;
}
.checkboxes label:first-of-type {
  margin-left: 0;
}