'Return the child XBlock identified by ``usage_id``, or ``None`` if there
is an error while retrieving the block.'

def get_child(self, usage_id):

    try:
        child = super(XModuleMixin, self).get_child(usage_id)
    except ItemNotFoundError:
        log.warning(u'Unable    to    load    item    %s,    skipping', usage_id)
        dog_stats_api.increment('xmodule.item_not_found_error', tags=[u'course_id:{}'.format(usage_id.course_key), u'block_type:{}'.format(usage_id.block_type), u'parent_block_type:{}'.format(self.location.block_type)])
        return None
    if (child is None):
        return None
    child.runtime.export_fs = self.runtime.export_fs
    return child
