You can not change what other people believe in, neither do you have the right to. That being said, your wife is the mother of her kids, and therefore she has the responsibility to show the kids what true freedom means. And if her ex husband is manipulating the kids, then you should encourage the kids to reject that manipulation for what it is. Teach them that spying on each other is an absolute no-go.

In the end this is a more or less perfect example to teach the kids the freedom that Jesus introduced into this world and show them the shackles that religion and religious people introduce to the world.