If the question is real. There is a very simple answer to the problem. Wear higher necked tops when with your grandchildren. That way they will not be able to put their hands down your sweater and fumble your breasts. After a few months they will; get out of the habit and you can go back to your usual tops.
I think the OP is making a mountain out of a molehill, so to speak. Whether you have had a mastectomy or not it is inappropriate for your DGC to be fumbling your breasts. You should have stopped it before it started. Just stop it now by the means suggested. No explanations are necessary.