Given what you say about your mother, I am not sure that it would provide the barrier that you seek.
Grandparents have a stake in grandchildren, that is a fact of life not only JWs. I have known of Witnesses with d/f’d adult children who still cared for the little ones one day a week.....with the parents permission of course, just taking the kid at the door.
Frankly I think your fears of kidnap are unfounded. The WTS would never embrace such a policy. Even if they wanted to, their lawyers would not let them.
I understand you don’t want your daughter indoctrinated with their beliefs. I guess you will just have to be firm with your mother , telling her to never say it in your daughters presence. A threat of not being able to see her granddaughter at all might do the trick.