Maybe we should do both. I definitely think we should do the child_rel stuff in some form. I hope that you put it into your current implementation, because if you don't retain that information in your data model, it will be much harder to upgrade to a standard that has explicit translate/localize settings.
But if we have the child rels but don't use parent id, then any query that gets back a bunch of cr_items will have to filter out based on child_rel. So this argues, if I understand the cr model correctly, for using your parent_id approach for every content item that is a translation or localization of another item.