OMG, gingernut! Your latest post put a different face on it for me! I don't blame you for speaking up in those 2 instances. Especially when I think of your GS sleeping on a bed with no bedding. Poor little guy!
I'm particularly worried about the kids being so dirty and smelly all the time. I take it they're not in school yet or, if they are, the school hasn't said anything.
I'm a little confused about DD. At first, you told us that if you try to talk to her about these problems, you get threatened with a CO. But now you say that in those 2 instances she listened to you and cleaned up. So maybe she your speaking up was working earlier but then she got tired of being told?
Be that as it may, if you're words aren't helping anymore and you really feel the kids are in an unhealthy, unsafe situation - and, I'm sorry but it sounds to me as if they are - then I'm changing my advice somewhat - I think you're only option is to call Social Services. I know you don't want to, but I don't see how you can just ignore this and I can't think of any other way to help those kids. Social Services may be able to do what you can't.
Maybe you want to wait until the health visitor comes to the house. Maybe they'll have a talk with DD and SIL if they notice these problems. Maybe they'll even make a report (they may be required to, I don't know). Then you would be off the hook and your GC would still get help.
I know you'll want to think carefully about this. If you do end up calling Social Services, might ask you if you would keep the kids for a while, as they're parents get their act together. Would you be willing to do this? Also, and if DD and SIL find out/suspect it was you, I know you may still get COd. But you'll know your DD's family is getting help and that you did the best by your grands. In my prior post, I know I was concerned about your providing a social outlet for DD and the kids, but now I'm thinking the children's health and safety are more important, especially if it's as bad as you describe. And I know you may be worried about their taking the kids away from the family, period. But as long as it's just a matter of cleaning up/keeping the house and kids clean, I don't think that will happen. I may be wrong, though, so again, I realize you'll want to give this a lot of thought.
Sorry that the advice here has been so contradictory - and now here's someone (me) who has actually changed hers! All I can say about that is please go with whatever rings a bell in your heart. My own heart goes out to you and yours once again!