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