It's great you're not losing them!
And the best "witness" you will be able to give them is showing them love and letting them see that you are still a wonderful man and treat your wife and children wonderfully!
Regarding Contact with the Kids:
They'll miss the kids too much to stop having contact with you. But if this happens, it is more detrimental to the kids to witness the type of shunning/abusive behavior of the grandparents. And if they choose to shun you, be sure to explain to your children the reason their grandparents are no longer in their lives. This will essentially innoculate your kids to never join the cult. And you could tell your parents this as well, "Mom and dad, by your shunning me, you are innoculating your grandchildren to never become JWs. So you're doing me a favor."
My kids witnessed the shunning of my sister toward me and refused to attend her wedding!!! Believe me. When kids see their parents being shunned and abused simply because they don't believe the teachings it has a VERY LASTING IMPRESSION.
My personal opinion is that your kids shouldn't be put on the spot of having to say yay or nay regarding meetings. You know it's not true. You know it's a cult. Why would you allow your kids to be exposed to this as young and impressionable as they are? Your parents may not pressure or coerce but the little snide comments from other well-meaning people in the hall could do the damage for them.
I would let my own parents know that they can see my children any time but they will have no "grandparents rights" as far as taking them to meetings, or any activities associated with the JWs, reading them JW material, talking to them about god, religion, etc. It's best to draw a clear line or later on there will be confusion. And spending time with them outside of witness activities is a privilege not a right. The second shunning comes into play, all bets are off.