This is just silly. Didn't Charles Schulz consider himself a secular humanist, at least at the end of his life? Maybe these folks should take a lesson from him.
I really don't see what harm is caused by kids seeing this play. If they come home with questions about seeing something different than what they believe, it would be a good opportunity for parents to explain to their kids something that all kids should be taught: in life people believe different things and it's important to respect people regardless of their beliefs. And we don't need to tear down every sign of something different than what we think.