Personally, I feel it is better to have a talk with with "grandmother in chief" for each family instead of a blanket email or letter to them all at once. I think expressing your concerns and feelings will bring you closer together and that's in part what the holidays are about.
I would be slightly ticked off if I got a blanket letter like this instead of a phone call to discuss gift exchange ideas, but that is just my honest opinion. I totally understand where you are coming from, but I think it should be a personal conversation.
This is my first thought. And you could do a letter, maybe, but I would reword it. I saw a gift idea letter once....I think maybe in Mothering? that was very good. It seemed like it did a good job of getting the point across without coming across harshly. I know a couple of the ideas it had was classic toys--something that was big from the grandparents day and a story to go along with it. I think that's an awesome idea. Another thing I remember it suggesting was an experience...a day with Grandparents at the park, a trip to the zoo, a memebership to a club...that kind of thing. I think that's awesome gift ideas too--plus no junk clogging up the house!