Oh Lollipoplove, I'm so very sorry to read this, and I feel your pain - 2 of my children stopped me seeing them or my grandchildren for some years after I separated from my husband, 1 of them was only 1 year old when this happened - so I don't have any bond with him at all.
Might going to counselling, like Relate help? You could say to your daughter that you know things aren't as good as they could be between you, and you'd like to try to make them better, if only for the sake of their daughter. (Although I have to confess that it didn't work for my daughter and me.)
If your D and SiL aren't willing to go to counselling with you and know how you feel, then I'm not sure that there's much that you can do about how they behave towards you.
With regards to your grandchildren, maybe buy a more modest present for their birthdays and Christmas, and give them little presents through the year to remind them that you are a part of their lives.
Likewise, when you are on a holiday or day out with them take lots of photos - ask your D/SiL to take them of you with the grandchildren, or do selfies - and then have them made into a little photobook, so that they have the memories to reinforce that bond.
From the way your post is worded, it sounds like they live nearby - could you set up a regular thing with the grandchildren - like them staying over with you or babysitting on a Friday or Saturday night - you get quality time and the parents can go out (you don't say the ages, nor how many, so this might not be possible).
On a day out, if you've bought the entrance tickets, maybe gently say at lunchtime that as you bought the tickets, it would be fair if they got lunch.
And maybe consider having some counselling yourself to help you manage your feelings about the situation - it really helped me. You'll probably have to go private, but what price feeling a bit happier?