I think it is great that you have a somehow good relationship and that you were strict with your "package deal."
I think it is important, that the grandparents have a good relationship to their grandkids, even if it means that they hear from time to time jw propaganda.
If you for example forbid free speech in those relationships I think this would backfire.
Make sure that your kids learn to critically think for themselves. Teach them that many people believe in differemt things, different gods or no god at all, and that's all right. Show them that before truly believing something it is good to look at the facts from various sources.
Tell your kids that they are free to change their belief a thousand times and that you love them unconditionally no matter what they believe.
If they come with a jw or bible story like "Noah's ark" you could ask them. "What do you think about this story? Why did Jehovah bring the flood? (Because people were evil). Hmm... do you really think all people were evil? And what about the little children, babies or grandmas and grandpas? And all the animals? Did the cute little cats (or choose an animal you child likes) really deserve to die a horrible death? And by the way, how did they all fit into the ark? .......
Be prepared to discuss with your kids like that about different bible stories (maybe check the bible story book, there are many questionable stories that show how cruel Jehovah is).
Something similar could be discussed with the theme armageddon.
Some great ideas how to help your child think critically and becoming a freethinker, breaking indoctrination you find in the book "RAISING FREETHINKERS - A PRACTICAL GUIDE FOR PARENTING BEYOND BELIEF." This book is a real jewel.
You can be glad, that your kids are growing up with you, who left this cult. In my situation it is more difficult to counteract the indoctrination as my little son lives mainly with my uber dub jw exwife and I see him 2 or 3 days all 2 weeks. But even like this I think I am managing quite well to teach him critical thinking skills. He is 2 years and 8 months and he learns so much and understands most what I tell him. So he recently came and said the name jehova. He thought I would be happy to hear him say that. I didn't react negatively. But at another instance I explained him that mummy believes in jehovah and I don't believe that our heavenly father is jehovah. Some believe in Allah others in no god. And that's allright. We can believe different things. Even as young as he is he seemed to understand, and that it is more important that papa or mama are kind and loving to him.
Concerning praying. Before the meal (especially when my parents are there) I say. "Let's just have 30 seconds of silence. This is just a very personal moment for everyone to think of something nice and beautiful or for thode who wish to say a silent prayer.
I wish you a happy family life with your kids AND grandparents.