Congratulations on your baby!
I'm going to offer a contrarian point of view based off of my own experiences. FYI, I was raised and baptized as a JW at age 12, and faded out in my early twenties.
None of my grandparents were Jehovah's Witnesses, and my parents didn't allow me to have much contact with them. My grandparents were vocally disapproving of JWs and my parents thought they'd turn me away if I had any contact with them. As a result, I never got to know much about them, their lives, or their life experiences. By the time I left the organization, they were all dead. I know my parents were doing what they thought were in the "best interests" for me, but I still resent them for not allowing me to develop any sort of relationship with my grandparents, regardless of what their beliefs were.
Your are in the inverse of that situation. You want to look after the best interests of your child by keeping them away from their grandparents who may try to influence the thinking of your child's young mind towards their religion. Of course your MIL wants to take your kid to the zoo. She probably thinks that's a great opportunity to teach a child about the wonders of creation made by a loving God.
Someday your child is going to want to know their grandparents. It may not be until they are grown up, but by then it could be too late. Until they're 18, it's your choice if you allow that to happen. I think the key is limiting influence while still allowing contact. Remember, that for many years, as Mom, you are the prime educator in your child's life, and they are going to listen to you over anyone else. If you educate and explain things to your child, helping them to develop their cognitive skills, and be a good example yourself, I highly doubt your in-laws will hold any risk of turning your kid against you. And someday, it may be 50 years in the future, your child will thank you for having the opportunity to know their grandparents.
Every family situation is different, because every person is different, so advice that works in one situation may not work in another. That said, I can say from experience it is no easy task as a baptized Fader, living with his "worldly" fiancee to have even limited contact with his parents. That by itself speaks volumes about your in-laws. Regardless of the intentially mean or argumentative things they say, it shows that the familial bonds are tugging on them equally to their spiritual vows. They also haven't gone to the Elders to get your son disfellowshipped, which means they are probably secretly covering over or leaving out facts about their son when others in the congregation ask them about him.
That is going to severely hurt their consciences, which means they are going to say and do things in their relationship with you that justify and rationalize their actions. This is the hard part, and I will explain.
My father has been an Elder for over 40 years. I've been disowned 3 seperate times by my parents over 7 years, being told face to face they never wanted to see or talk to me again. The first time was a deliberate effort to scare me back to the meetings after I was inactive when I told them I was going back to college. The second time was when I let them know I had been dating a "worldly" girl for several months and wanted them to meet her. The third time was when they figured out we were living together. Each was followed by months of no contact.
A few months after the second disownment, I went by their house to say "Hi". My mom opened the door and said "Who are you?" I said "I just came by to say Hi and see how you are doing." She looked at me, then looked down at my crotch and said, "I don't know who you are, and I don't know where that has been, and neither of you are welcome here."
I cannot recount all of the intentionally hurtful things that were said by my parents. I cannot count all the times I've been told "I wish you had never been born." At each step of the way I could have said "F%uck it!", blown up on them, and walked away never to speak to them again. I do have the self-esteem and self-confidence to do that. But I didn't. I recognized that just by keeping contact with me, their love for me was equally as great as their love for the watchtower, who told them to shun me altogether. After 50 years of indoctrination, they still talked to me, in violation of the rules, even if it was just to yell at me or tell me how horrible a person I was.
And slowly, very slowly, things have changed. I did what they could not do. I turned the other cheek. They were constantly baiting me to say something or react in anger to give them justification for how they treated me. And I didn't give it to them. We have slowly achieved a stalemate. They don't talk about doctrines or how I need to come back to meetings anymore. They will occasionally have dinner with my fiancee and I. They don't approve of me living with my her but they no longer berate me for it other than the occasional "When are you guys getting married?". I'm still not sure if I can get them to attend when it does happen, but I'm working on it.
The point is, you have to ask yourself how important it is for your husband, your child, and yourself to have some sort of relationship with his parents. There will never be some single confrontation or argument where someone is proved right or wrong. But the fact they contact you and want to have a relationship with their grandchild says you have a chance. It's not an event, it's a process. And for any hope of success, you're going to have to ignore a lot of the argumentative things they say. You will have to prove their preconceived notions wrong by your actions.
If you really want a relationship with them you have to choose your battles. It's a pyrrhic victory to win a battle but lose the war.
