docs: @UiThread
      public void notifyChildInserted(int parentPosition, int childPosition) {
          int flatParentPosition = getFlatParentPosition(parentPosition);
          ExpandableWrapper<P, C> parentWrapper = mFlatItemList.get(flatParentPosition);
  
          parentWrapper.setParent(mParentList.get(parentPosition));
          if (parentWrapper.isExpanded()) {
              ExpandableWrapper<P, C> child = parentWrapper.getWrappedChildList().get(childPosition);
              mFlatItemList.add(flatParentPosition + childPosition + 1, child);
              notifyItemInserted(flatParentPosition + childPosition + 1);
          }
      }