A lot of food for thought here....
Rock and hard place, Stella! I totally feel for you! I know you miss DD and family and would love to live near them. But once you're there, no doubt, you'd miss DS and his kids, as well. So this comment about DS' kids caught my attention:
"Their own mother is a distant figure and totally uncaring, so I've really been the female influence in their lives for all these years."
So if you just up and move away, it may feel like a "second mother" distancing herself. I couldn't do that to them. I wouldn't rush to move for this and some of the other reasons that have been mentioned.
Then again... "... my son often talks about his dream of moving to the Gold Coast since he is a surfer. I'd then be left with just me and DH , both getting older."
So I woudn't brush off DD's offer either. As PPs (previous posters) have suggested, I would make the transition slowly, going to the granny flat for a holiday or two, at first, to see how things go, then maybe for the summer, etc.
As Meeyou says, "It could be heaven or it could be hell." But you may have a chance to try it out and see. And, meanwhile, you won't be abandoning DS' kids while they still need you.
Please think it over and discuss it some more w/ DH. As a PP said, no need to rush into a decision.
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