Wow! (not me - the situation!)
I really like MK's tough love advise but you keep saying you don't like conflict.
You can have conflict with or w/o having words, with or w/o tough love, talking, even understanding why. If you know why but the situation doesn't change, you're still going to be upset.
In really tough situations, I like to try to work AROUND the problem and when something has been brewing for as long as your predicament, I think you might find that the easiest approach.
You say you've already asked your son and got nowhere. Put that aside for a moment.
You say you want your grandchildren to get to know you. Here is what I would do(in no particular order):
I would get a journal for myself, a scrapbook, some stationary, a camera and/or video cam and a second journal.
Instead of calling, which hasn't brought you any sense of resolve, that you are able to get through to your grandchildren, I would send them greeting cards for b-days and every major and minor holiday. I would include photos/videos about your world and possibly got with trendy things such as stickers for the envelopes, based on their age.
I would journal about how it makes you feel to be pro-active, what losses you still feel and you can also snail mail with a signature required. You'll find out who is taking in the mail for each card or letter you send and if you get a reply to some but not others, assuming you'll be keeping track of dates and signed card you get back from the P.O., it might help you have a better idea if there's a pattern or which one of them is really at fault, behaviorally, IF you get replies. This would be a non-invasive way to do some detective work at the same time as making a connection with your grandchildren. They may be forbidden to reply but it would take a lot of heartlessness to deny them photos and letters/cards and I would go very slow starting with a hello card and photo of you and just signing it. Build on it from there and make photo copies of everything. The scrapbook would hold all of the copies.
The second journal, I would start when you feel back in your own skin, so to speak/not overwhelmed by a sense of loss over this. I'd journal what you want your grandchildren to know - daily. Something, if nothing breaks the ice, you have dated and can give to them when they're of age. You'll have a better idea of what to say, if anything, about their parents, once you have some signed cards from the P.O. and I'd start this a week before Christmas - keeping the status quo, will make it harder for their parents to back out of the only in-person visit you have - they won't cancel over a Christmas card. But, during that time, you can ask your grandchildren if they got it or send two separately and include a small monetary gift to not rock the boat with the signed cards. Their parents won't think twice about it. (anyone who sends something monetary through the post office wants to make sure it has some protection - after that - you're simply doing the same thing because you want to make sure the photos don't get lost in the mail, right?) --- By sending two cards (I'm kinda guessing or overlooked something - you said you have 2 grandchildren?) you have two children to ask so if one doesn't get your card, you'll know by the P.O. card who signed for it.
After Christmas it will be like a crap shoot if your grandchildren reply or not but, imho, starting to snail mail around a holiday their parents are willing to comply, would make it easier for them to say yes and harder for them to say no to adding a "please write back soon" to the first cards you mail after the New Year. What you don't want to do is buy them stationary for Christmas - it's too obvious. Kids can draw stuff and use their imagination w/ plain paper. The point being simply hearing from them a bonus to letting them know about you.
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Depending on how that might go - worst case scenario your snail-mail is returned unopened (unlikely at Christmas time), when YOU feel comfortable with YOUR journal, write your son a letter and mail it to his place of work. Share some of the things you've discovered you feel about not having HIM in your life - I wouldn't mention his wife and if nothing gets mailed back to you, I would simply compliment him on being a fine dad to let his children see your photos. I'd actually postpone this letter until you've worked your way up to videos or possibly make a video saying what you would in a letter and include some baby photos of him. The latter might provoke a small amount of guilt but, in presenting it, tell him you thought he'd like to show his growing children (bolding because that will give him the reality that those kids won't be under parental control forever and have questions - make it easy for him to keep the door open and request his confidentiality because you feel this time(his baby pictures) is part of your former life relationship w/ him. Tell him as his mother (whether you could or couldn't do that for him is irrelevant) that you realize it is important for him as a dad, to have that opportunity as a dad. Totally non-confrontational and a gift you'd be giving to your son.
If all of that fails to turn him around, you've not only done everything you can as a parent and grand-parent, given yourself copies of your efforts but, you've just built yourself a mountain of evidence if you decide to take him to court. Love always wins - whatever you decide.
Not sure this is the kind of answer you're looking for. I'm just telling you what I would do and do with it.
Take it for what it's worth.
I hope things get better for you.
