Based on your example above, you can create a mapping to use as an import mapping when you GET. After the GET, you'll need to do some retrieves and populate a separate entity with the results you want to display in a datagrid.
For instance, suppose your elements above are called FirstElem, SecondElem and ThirdElem and that ThirdElem has sub elements called ThirdElemSubs. You would create a mapping to use in the Call Rest action. Assuming the mapping starts at Root, you could retrieve the three top level elements and then retrieve the sub elements from ThirdElem. You could then use a list aggregate action to count the list of ThirdElemSubs. Finally, you could populate your new entity with FirstElem value, SecondElem value and ThirdElemSubs count. The new entity could then be displayed on a page as you described above.