'Returns a list of XBlock instances for the children of
this module'

def get_children(self, usage_id_filter=None, usage_key_filter=None):

    if ((usage_id_filter is None) and (usage_key_filter is not None)):
        usage_id_filter = usage_key_filter
    return [child for child in super(XModuleMixin, self).get_children(usage_id_filter) if (child is not None)]
