Well, I would never do anything with my DGC that their parents had strong objections to and, as far as far I am concerned any grandparent that did, is way over the dangerous secrecy boundary.
I am talking about the little secrets and conspiracies, which is what makes up the vast majority of grandparent/grandchild secrets. DGD helping to put cherries on the cake, and popping an extra one in her mouth, or the one described in my previous email, staying up a bit later than normal to watch a DVD, when with us, things that, actually do not matter a toss if parents know, and lets face it they usually leak out anyway.
I do think children quickly learn the difference between, what I describe as complicit secrets about trivia and ones where there is fear of either the person insisting on secrecy or fear of how the person who mustn't be told will react if they do find out.
Good heavens, children love secrets, it is in much of their play. When DGC visit, secrets abound. DS looking for his socks and DGS whispering in my ear that he left them under the table and he mustn't be told, because of the fun of watching the search. Another secret, DGD tells me that DGS is making a card for grandpa, but he mustn't know until its finished. There is an enormous gulf between these sort of trivial confidences and serious dangerous secrets.
The vast majority of grandparents are well aware of the difference and do not need to be told.