I'm so sorry you're going through this ethelbags. I think you've gotten a lot of good advice here and I hope it helps. I want to add that I think part of the problem here is that you're watching DGD very often - probably cuz DD works? - even through her dinnertime. You want to be a fun GM, but when we take care of our DGC a lot, I think we have to do some disciplining. I watch my own DGC often, so I know how hard this can be for a GM. But my DD and I both agree that it's necessary.
That being said, IMO, it would be ok for you to offer your DGD her favorite foods, every time, for a while, as long as we're not talking about chocolate and other junk foods. I don't think either you or DD should make too much of it if she doesn't eat, If she's really hungry, she will.
I agree with you that attention is good - but the positive kind, like when she sings for you and so forth - not the negative kind, like when she won't eat.
Another thought - I don't know how many hours DGD is in nursery, but if she has lunch and snacks there, maybe she's truly not hungry at her usual dinnertime. Maybe it has to be adjusted? If DD is ok with it, maybe you can just wait until she says she's hungry for a few days and see where her new eating time may be? Just a thought...