This situation you described was not exactly top of mind, I am sure, when you were drawing up your divorce contracts.
I totally understand your desire to minimize your children’s “confusion,” particularly during this post-divorce transitional time. It sounds like you and your ex-husband both committed to raising them Jewish, and you certainly have the right to express what you feel would be unsettling during this time. But on the whole, I am not sure the request to keep them away from doing anything Christian with his new girlfriend is either reasonable or even sustainable over the longer term.
As a child of divorce, I know that divorce comes with sacrifices; as a person married to a non-Jew, I understand the complications of introducing non- Jewish rituals into your collective lives. I can say with confidence that you will not somehow compromise your or their Judaism if they are to spend time with a person who observes another religion. In fact, your husband and you would be teaching them to appreciate and navigate the rich diversity of voices in the world.
I think one way you may approach your husband about this is to suggest that he frames it as a cultural experience; he is not injecting another religion into their lives. Cultural experiences can feel less threatening for both adults and children, particularly during times of instability. They are less emotionally charged and can provide room for learning opportunities for all parties.
I also think that for pre-Bar Mitzvah age children, hearing diverse voices and the exposure to how others are spiritual can help strengthen their own identities. With a strong Jewish foundation at home, the new cultural experiences in their lives can only further serve to shape how they view the world- and themselves.
Paula Sinclair, most recently of the Jewish Women’s Archive, loves to write, collaborate and teach about contemporary social issues in the Jewish community.