To me the dangers of exposing ones children to practicing Jehovah's Witnesses are on par with exposing them to a dumbass, and hell yes they will read them the literature, many is the time the subject of articles in their publications were encouraging just that, covertly indoctrinating and manipulating children while been told NOT to by non practising or "worldly" parents friends and guardians.
im not saying don't let your kids see grandma, but don't leave them unsupervised and I certainly would nt make a habit of regular unsupervised over night stays.