def addFilteredChild(self, parentItemId, childItemId):
        parentToChildrenList = self._filteredChildren.get(parentItemId)
        if parentToChildrenList is None:
            parentToChildrenList = list()
            self._filteredChildren[parentItemId] = parentToChildrenList
        self._filteredParent[childItemId] = parentItemId
        parentToChildrenList.append(childItemId)
Adds the given childItemId as a filteredChildren for the
dataset/ETHPy150Open rwl/muntjac/muntjac/data/util/hierarchical_container.py/HierarchicalContainer.addFilteredChild