I can see that for you Red, this is something you dont like the idea of , for yourself. So are you putting your own thoughts into how you interpret how the children are feeling? As other have said it is a long time in the future and much may happen to alter their plans. You cannot really influence what they chose to do, it is their lives and their family. But you could show a calm and accepting attitude to the children, that all life is change. Some things we can influence others we have no chance to change, whether this is work or health or who knows what.
The best lesson to learn is to accept that nothing remains the same for ever, and to be prepared to go with the flow, learn as much as you can about the possibilities there are, and realize that whatever comes or goes, you will be strong enough to deal with it. Just think how the children have changed as they have grown. They would not want you to hold their hand across the road nowadays would they? But as little children they were taught things like that to keep them safe, until they were able to deal with it themselves.
Having lived abroad and in Britain, I think there is a lot of good things to be learnt from moving about. Understanding a bit more about the world and not assuming that the way you live in one place is the same for the rest of the world, learning languages and finding out new things, new friends, is all very worthwhile and gives you a wider view of things and of course, if you have learnt other languages and become a good speaker and writer you also have new opportunities in lots of places for interesting jobs.
So, if you can help them to look forward to new things, and at the same day, you will show them that you are their granny and will always be there for them whatever comes or goes, you will do them a favour, and I think at the same time help yourself.