Well some things are inevitably going to change, so while you will find travelling a lot gets too much effort both practical and emotional, your grandchildren will be growing older too. So I would be looking at making your own relationship with each grandchild, so this might be something like the whats app idea, or that you have a phone call with them regularly, but not so they think its something they have to do rather than want to. In the meantime you can be looking for interesting postcards and sending letters directly to them. According to latest info , postcards are coming back into fashion. So you might have something you share - ranging from silly postcards or ordinary blank cards you can fill in yourself, or if they love cats or horses or whatever pick up on that . develop something individual with each child, and even with the latest phones etc, you could find that they enjoy receiving their own letters. Have any of them learnt to play something like chess or bridge? as they get older, so you could set up a bridge tournament where you do a move a day or whatever suit. Then this will mean as they get older and can come and visit you on their own you have developed something to share that is special to you, where you might go swimming or fishing or whatever appeals.
So I would suggest you think about what might be the special thing you could do with a child and see if they like your suggestion. I would simply , without making a big deal of it, say that now you are not able to do the sort of travelling you have done in the past, and could make it less about the family by saying that you are not going to x or Y this year, too much effort. If it is not tempting fate I might be prepared to slightly embroaider the truth and say long trips now make you feel, sick or dizzy or whatever, so you are looking at other ways to do things. You could also preface this by saying that someone - preferably that they know - has stopped going to wherever now and you can understand this as you are also feeling the same, and have decided that you can no longer do long trips in one go etc. This makes it a statement of fact, not specifically aimed at then, but that way you have reminded them that you are gettting that bit older and they should not expect the same ability to go at their beck and call and quite frankly It is about time they thought a bit about you and did the travelling. Book your clubs and so forth and simply say oh dear you would not be available for this that or the other, but you could suggest that instead of you going to them or even them coming to y ou , you could look for somewhere half way that is easy for you to get on, and suggest that you meet up for a long weekend or whatever . You are being totally reasonable and they are not. Time they made the effort. if the adults wont, keep trying to keep in touch with the grandchildren, Good luck and hope you find something that suits you