What a wonderful Grandma you are!
Here are a couple of idea. Since you are so busy some of these ideas can be done all at once and then sent out over time.
1. Purchase some new books and record Grandma reading the books.
2. Record a song from you to them for mom to play at bedtime. My Grandmother always sang to us when we stayed with her, great memories.
3.Do you have a computer with a camera? If so, and Mom has one too, you can do projects, even coloring or playing with play dough on camera at the same time.
4. You can send them a hello card and mail it. Children love to check the mail
5. OR you could send them e-cards really often. Kids love to check THEIR email!
You've had kids so you know that the only thing that's going on here is that the kids are at a place in their development where people they don't see everyday seem like a stranger for a little while when they come together again.
All you need to do is be part of their lives in some small way everyday or every couple of days so they remember. They will still have a hard time remembering when they see you, but now they will be able to relate to you, the songs and the stories.
Good Luck, The Mommie Mentor, www.proactiveparenting.net