My situation is a little different and more complicated but I understand the jest of your question. I am not a JW but my wife was baptized about 2 years ago after studying off and on with the JW's for 17 years!!! She hesistated that long because of the complications she knew it would bring in our marriage as we have never been spiritually united but she knew that this would cement that division. My wife's parents are both harcore JW's but that was not the case when my wife and I first got married. It took time but now both of my wifes parents, sister, 2 cousins are all hardcore JW's. We live in Ca. and my wife's family live in AZ, thank God, but even then we still have issues. My wife's mother is very very hardcore and manipulative in many ways. Once when we were visiting them about a year ago, we were at a "get together" at my wifes JW cousins house when my mother in law wanted to leave early because she was tired. My 9 year old son asked if he can go with her because he was bored and wanted to swin at my mother in laws house. I said ok. When we got back to their house a couple of hours later, my son came to me and repeated a scripture verse he memorized in the 2 hours he was alone with his grandmother, I think it was in Revelation chapter 20 about how Satan was cast down to earth. After he stated the scripture verse he said "see dad, that's why bad things have been happening ever since." There have been other examples here and there of my mother in law doing things like this over the years. They are not always "over the top" in the things that they say to my kids although there have been a few incidences, but rather they are very subtle or sneaky, depending on how you think of it. You as a JW should know that the spiritual divisions that they cause in families only validates their belief that they have they true faith because of Jesus's words in Luke 12:49-53
“49 I came to start a fire on the earth, and what more is there for me to wish if it has already been lighted? 50 Indeed, I have a baptism with which to be baptized, and how I am being distressed until it is finished! 51 Do YOU imagine I came to give peace on the earth? No, indeed, I tell YOU , but rather division. 52 For from now on there will be five in one house divided, three against two and two against three. 53 They will be divided, father against son and son against father, mother against daughter and daughter against [her] mother, mother-in-law against [her] daughter-in-law and daughter-in-law against [her] mother-in-law.”
Here's the bottom line, I believe that what you seek, keeping peace while not having your kids sucked into this religion, is not really possible because if they really are hardcore JW's as you say,they most likely will not let it be possible. I don't know your family but I don't have to because if they are hardcore JW's, they will follow the teachings and you know what those teachings are. Don't be in denial about this because the consequences are so great. I was in denial about it for years and I have paid a huge price. I'm still trying to balance things as much as possible because of my mistake of trying to keep peace. Keeping peace means that you back down or submit to their subtle indoctrination of your kids because they sure wont submit to you, at least not if their going to follow what they are taught. If you try to keep peace in this way, it may take a toll on your mental health if you don't nip it in the bud right away because if you don't, you will be setting yourself up to walk on egg shells forever.
You have a chance to nip it in the bud right now but only if you get over this denail of thinking you can keep peace while being firm to keep your family from influencing your kids with their religious beliefs because that's what their mission is... Do you get it??? You will have to be firm in leaving no doubt that you will not tolerate them teaching their beliefs to your kids in any way shape or form and as long as they are willing to agree to that, then you can be amicable with each other and still have a relationship. Put the ball in their court but they have to play by your rules. That's how firm you have to be so that you leave no doubt where you stand. If you give a hardcore JW any wiggle room, they will wiggle and play until you leave no doubt, then they have to decide what they want to do. Be prepared for what I believe is the inevitable of them either slowly losing contact with you or maybe they will leave no doubt with you as well where they stand, in which case the break up will be more harsh. I wish I could be more positive about this but I know many JW's and once you give them notice that you know their beliefs and don't believe in them and that you don't want them spreading their beliefs to your kids, things will change.