Perhaps one idea might help. Whilst I know that we all have things like emails and facebook etc , I think it would still be enjoyable to receive letters addressed to you specifically as a child . So depending on how many grandchildren you have , you could set up a correspondence with each child that is special to them. Not expecting them to reply by letter, but be happy to hear back from them by phone or email or whatever.
So the idea is that you think about what they specifically like individually and respond to that. So if one likes steam trains or tractors you might send the odd postcard of a special one, or plan to meet up at a station in the future. Then you let them know what you are doing and send the odd picture of the cake you made or the things in the garden etc., showing them that you are constantly thinking of them , even if you are not able to be there in person. You might collect some seeds from your garden for them to plant in their garden, ask what they are reading and if they belong to any group such as scouts or swimming group etc. Even if they are mad about a particular pop group you might find pictures of them they might like etc. It really does not matter which area is their interest, it is about you developing that relationship with them individually so that when you see each other you have things to catch up on and they know that you care about them and think of them often. I dont know how old your grandchildren are but I do know that getting your own letter or message is still something special especially to a junior age child who will enjoy that personal thing. If your writing is not very clear , even if you type it and then send the typed letter with just love added at the bottom so that they are able to easily read it themselves. You can keep their letters is they send any to you, and I would put a note of the date they arrived on them and a long time in the future they will enjoy reading their own letters and remembering things from years before.
The idea is not to see it as a chore, nor make it a set specific time to write, but when there is something to share or something happening that you want them to know. This way you will be part of their lives, but in a different way to the other granny. When you are able to see them you will be up to date with all their doings and have things to talk about and share. You could make a scrap book for each child , where you make a note about things you did, possibly add their letters to you, add notes about places you visited together etc. Then when they are much older you might be surprised at how much they enjoy looking at them and remembering old friends and things you did together.
As they grow up they will perhaps be able to visit and stay with you for a day or two, or another possibility is that you might do a bit of research and see if you might be able to meet the family on holiday somewhere, perhaps staying bed and breakfast somewhere near them or sharing a holiday cottage or something with them, if money allows.
These days many people live very far away from their families and life is definitely changing for many of us , so that the old ways do not work now. Finding new ways of keeping in touch is the way forward I think. Hopefully something like this will make a bond that will last for years , but even if it does not go that way, you will feel you have made every effort to show them how much you care about them and are part of their lives. You also might do a bit of family research and perhaps write your life story down for them. When we live close by, we hear different tales of the past as we go but they miss the opportunity to hear about your life that way so writing about your own school days and friendships and houses you lived in etc can be of interest in the future to them and give you something that you can enjoy writing about looking back over your own life. Perhaps you might even get in touch with an old friend or two and meet up for coffee or lunch together and enjoy that too. Wishing your all the best