I think you are in dangerous ground here. For starters she will never stop telling your kids about WT crap. I was brought up in the truth untill I was 9 years( not very strongly) but it NEVER left me or my sisters..............once it gets a hold of you its very difficult to shake of the superstitions and fear of the big A....................I never stopped believing and then when I was 26, there was a knock at my door.
Twenty years later im just finding out that it was all rubbish. Dont let your kids be part of this......you need to deprogramme them NOW.
You need to tell them its not true...........its not what the bible teaches...........you must do it strongly today.
You can even tell them that granny is wrong !!!!!!!!!!!!!!!!!!!!!!!