I am not a Christian and I would love to participate in a nativity play :)
But I would hate it if I HAD to. So no, I don't think that anyone's kid should be forced to do that. Even if the kid is Christian or simply undecided. Religion should be voluntary.

But this post is not really about religion, is it? It's about people coming to live in a country totally different to their culture, and expect to be allowed to live there according to their own rules and laws.