It's heart wrenching. I went through a similar situation with my daughter. It's hell. The best day of my life was when my daughter turned 18, and her dad couldn't use her as a pawn to punish me. She wants nothing to do with the Watchtower now. Her in-laws go to a fundamentalist church, and the service makes her physically ill. She can only go to liturgical churches, where the focus is on worship, not evangelizing and/or judging.
Take one day at a time. Be sure the kids understand that you love and accept them for who they are and regardless of their choices of religion. Be sure they have a good time with you, and are going to have good memories of their time with you when they're older. I hope you don't talk to them about what's wrong with the WT. They'll figure it out by watching behaviors.
Hang in there. It'll get better with time.