Ok so how often are we talking here?
Are your dc happy and wanting to see their gps?
Is there any reason the gps can't come and collect the dc themselves or visit at your house? Are they very old, frail, have no transport...? I it impinging significantly on your time with the dc?
If you and your ex were still together would you feel he had to do all the running around re this or would you have made some effort - had them over, dropped the dc to them, invited them to the zoo/theme park with you, asked them to babysit...?
My ds's dad doesn't acknowledge he has a child and wants no contact. His parents are very upset at this and have always maintained contact despite this. They live abroad but regularly write, send parcels and presents, email etc. My ds has benfited enormously from knowing he has an extended family who care and love him. Most of his clothes come via them, and they lavish him with gifts at Xmas and Birthday (I know probably some of it is guilt at their son, but isn't that what gps do anyhow?)
Why should they and my ds miss out just because their son is an ar*e!? From my point of view it's very important for my ds to have this link and so I do keep them up to date on his progress, send photos and ensure he writes and emails them too. If they lived in this country I would be making the effort to ensure they saw him regularly too. But I know they would more than likely be making the effort to have this contact and visit, despite their own son's hostility to it.
So from my point of view I think it does rather depend how much contact they want and how often and what you would need to do to facilitate it. Are we talking about them wanting to see their gc ever week, living a distance away, and expecting you to drop them over and pick them up each time. Or are they simply wanting to see them in the holidays, maybe have them over for treats, sleepovers, days out. You would be getting a break at these times too possibly. And how about asking them to babysit so you can go out...? It could work to yor advantage in some ways too. Plus your dc will be maintaining a very important link to their extended family.
I would perhaps respond to them saying that you are of course happy for them to see the grandchildren and have they spoken to your ex re this. It is after all quite likely they have tried and also not got a very satisfactory response from him.
So their only option is to approach you. How much are you willing to do to facilitate this? Bear in mind that one day you could be the grandparent struggling to see yours grandchildren if your dc's marriages don't succeed. How would you hope to be treated?
Such things are never easy, but durely it's a good thing they want to stay in contact despite their son. Look at it from your dc's point of view.
Good luck