Set children on MoreRecursion objects.

This ensures that the json representation will have valid name,
id, and children.

r2/r2/models/builder.py

@@ -780,6 +780,7 @@ def get_items(self):
                                               parent_id=parent_id))
                 else:
                     w = more_recursions[parent_id]
+                w.children.append(comment_id)
                 more_recursions[parent_id] = w
 
         timer.intermediate("pick_comments")