The sooner you take the control of your child's upbringing away from the grandparents and give it back to yourselves you will feel so empowered. You already know the answer to this question and that is that you should prevent your child from attending any meetings with ANY jws.
Jw Grand parents are hard wired into bringing their offspring into the cult, no way around that fact. They will look you in the face and promise not to share any WTBTS propaganda to the child and as soon as you leave, it's on. They cannot help it,..but it will happen.
You have to figure out if you vslue your child's future and his sanity more than you value pissing off your parents/inlaws.
If and when you feel comfortable explaining that you were duped I would draw out a stick figure of a child, parents, Grandparents, elders in that order.
Explain how sometimes parents "mislead" the child into believing that their really is a Santa Claus And eventually the child finds out that they were not told the truth about Santa Claus. Then go on to explain that sometimes even parents and grandparents are told stories for years about certain things that are not true. I'm sure you see Where I'm going here.
Bottom line, get that child away from those people.
NJY