docs: @UiThread
      public void notifyChildRangeInserted(int parentPosition, int childPositionStart, int itemCount) {
          int flatParentPosition = getFlatParentPosition(parentPosition);
          ExpandableWrapper<P, C> parentWrapper = mFlatItemList.get(flatParentPosition);
  
          parentWrapper.setParent(mParentList.get(parentPosition));
          if (parentWrapper.isExpanded()) {
              List<ExpandableWrapper<P, C>> wrappedChildList = parentWrapper.getWrappedChildList();
              for (int i = 0; i < itemCount; i++) {
                  ExpandableWrapper<P, C> child = wrappedChildList.get(childPositionStart + i);
                  mFlatItemList.add(flatParentPosition + childPositionStart + i + 1, child);
              }
              notifyItemRangeInserted(flatParentPosition + childPositionStart + 1, itemCount);
          }
      }