'Instantiate an XBlock of `block_type` with the appropriate set of children.'

def _load_block(self, block_type):

    self.parent = ItemFactory(category=block_type, parent=self.course)
    self.children_for_user = {user: [ItemFactory(category=child_type, parent=self.parent).scope_ids.usage_id for child_type in BLOCK_TYPES] for user in self.users.itervalues()}
    self.all_children = sum(self.children_for_user.values(), [])
    return modulestore().get_item(self.parent.scope_ids.usage_id)
