I agree with both streams of advice here, Wannabe. I never contemplated being a grandmother, taking the view that that was definitely something that was a decision for other people, and that nothing I did would have anything to do with the outcome. I sometimes feel embarrassed when Mums put pressure on their DC. After all, we don't always know what is going on, it isn't our business to know all the background.
I have a toddler GS now, and DD and DSiL are wonderful parents though GS arrived later in life. I love my role as GM. Perhaps even more for not thinking about it in advance!
Sadly, even if you had been told that your DS was hoping for a family, it doesn't always happen, so you would then have even worse disappointment, and your poor DS and his wife to comfort, too, as others have said.
If you just love being around small children, I am sure your local primary school would welcome help with reading, or stories, or talking about your life, if you enquired. I know they are not 'yours' but they can be delightful just the same. I have known someone who did this who was called Granny by the children, as some did not have any GPs. Make life enjoyable as it is and if things change, you can be delighted, but not get depressed if they don't.