Just calmly explain there is a dark side to this religion even if Granny is lovely. Emphasize it messes with kids heads. And any local congregation paedo will not be pointed out to Granny either.
Despite gran's protestations to the contrary - in among the dancing trolley carts and happy smiley people he will be subtly taught that the downside to his living with Pandas in paradise is that all his kiddie friends will die at Armageddon. And his non JW family ...