Aztec,
I thought what Steve Lowry said was excellent:
what you really want to do is modify his behavior. Practically anyone's behavior can be modified.
Your father is really overstepping his boundaries (most JW's do, especially when the Society tells them it is their "responsibility to "save" their grandchildren's lives). If your father does not respect your boundaries, especially if he repeatedly oversteps them after you have gently reminded him, then you need to make it clear that he will not see his grandchildren until it stops. At the very least, not without you being present. This may sound drastic or cruel, but you won't have to do this for long. He will come around. By allowing your father to overstep his boundaries without repercussions, it negates your authority as a parent. Is that the message that you want to send to your children? You are ultimately responsible for your children. If you do not want them in a mind screwing cult, then you need to be proactive.
If you really don't want to destroy your father's faith, but just want to have him stop "witnessing" to you and your children, than don't undermine his faith with proof that his faith is false. Setting the boundaries accomplishes the goal of having him keep his faith AND his boundaries. My 2 cents.
exjdub