I don't know their full situation, of course; they may have debts you don't know about, for example, that would make it hard for them to move right now. If this is the case, they ought to make you aware of it.
But from what you say, it not only wouldn't be wrong for you to suggest they move–it would be right. This is true partly for your sake; it's not wrong for us to take steps to safeguard our health. The Bible says, "Each of you should look not only to your own interests, but also to the interests of others" (Philippians 2:4). In the past, you looked out for your son's interests; perhaps the time has come to begin looking out for your own welfare also.
But you also need to suggest your son move for his sake and the sake of his family. He needs to be responsible and provide for them, and he also needs to be a good example for his children. Situations like this can be awkward, but begin by asking your son what his plans are–and if he doesn't seem to have any, you may need to be more direct and suggest a deadline.
At the same time, ask God to help you be a good example to your grandchildren. Pray for them, and help them understand that Jesus loves them and wants them to be His followers.