Wow, I'm sorry to hear this. What is the reason for this? Is it because of slight theological differences that your child and spouse take very seriously? If so, then I would probably respect their request and try to work things out with them. On the other hand, is it because your child and spouse have given up the faith altogether (say, by becoming a mormon, atheist, or secularist)? If so, then I would probably talk to your child (and spouse) a bit more. Think of something that might convince them to change their minds. For instance, they might find it persuasive if you tell them that the world won't unanimously agree with them, and their children are going to have to learn to be confronted with new and unfamiliar ideas. Or, you could simply suggest that different people believe different things, and that you believe that Christ is so important that you simply must talk about him with those whom you love. Etc. In the end, if they are so hostile to the faith that they effectively want you to pretend to be a secularist/atheist when around their children (even after talking this out with them), then you would need to become very creative in how you make an impression on your grandchildren while making sure that you don't cater (in the least) to their atheistical requests.

I don't know, though. I'll be sure to pray for you in this situation.

Yours,
John

"He that hath light thoughts of sin, never had great thoughts of God." ...John Owen