children

I am a single christian dad who has been through a horrible divorce. I have kept my kids learning about christ and now the ex wife has made a choice to become a Jehovas witness and is bringing the kids to there building, I won't call it a church, and trying to sway there decision to go with her every Sunday... How do I address this with the children? She won't listen to anything I have to say, but I feel I have to say something to the children and I don't know how to do it? Need some help on this one!