That's what I'm thinking, that the grandparents were introduced to set up them taking custody of the two kids. Dexter might come to the conclusion that it's best for the kids too, since he might consider himself "poison" or something. He's got parental rights with Harry, but I don't know what he could do about Astor and Kody.
I don't think Trinity could have told Rita much, if anything, either. For one, he didn't know about Dexter's dark passenger. For all he knew, he probably still thought Dexter was just shaking him down for money alone. And, I would imagine that Trinity was caught up in his whole ritualistic zen stage when getting naked and in the tub with her. The times we've seen him in his rituals, he was really into them and not exactly making too much sense.