Unfortunately this is a common problem today -- one that would have been almost unknown a few generations ago. But times change, and increasingly families find themselves separated by great distances. This means they must make a special effort to keep in touch -- which is what I hope you'll do.
Although you seldom see your grandchildren, go out of your way to keep in contact with them, to let them know you love them and care what's happening in their lives. Call them as often as you can; send them e-mails; remember special occasions like holidays and birthdays. Write them letters also from time to time; getting mail is still a special treat for most children. You may not hear back -- but that's not the point; the point is to let them know you love them.
Then ask God to help you make use of the time you do have with them -- not just to have a good time, but to encourage them spiritually. Will they see Christ's love and joy in your lives? Will they come away realizing that Christ is important to you? Consider giving them a children's Bible storybook also.
Most of all, pray for them and their parents. God loves them even more than you do, and He can do what we can't do. Remember Samuel's words to his people: "Far be it from me that I should sin against the Lord by failing to pray for you" (1 Samuel 12:23).