There was a discussion on one of the listservs I belong. The question was about kids yelling out "that Santa doesn't exist" or something like that, when reading a Christmas story. The poster wanted others ideas on what to do and say, so this is what I shared: I usually say something about not everyone believes all the same things. I assure the children that I disagree and do believe in Santa. Or I say, that's ok that you think that. And then something that focus on the book or story, like let's see what happens next, and just go on. If it becomes a discussion between stories I usually say things like there are a lot of fun stories about things that some people believe and others don't. It's a great opportunity to talk about all the different things people believe in, and that because we don't agree on everything doesn't mean we can't get along or it would be boring if we all believed all the same stuff. It depends on your audience, age and where you are. I don't have a problem with discussing in a 'christian' school setting about the 'real' meaning of Christmas, because they will do most of the talking and relating it. It's also a good idea to discuss this with the teacher, parent, etc before to see what they talk about in class or home, etc.
Sometimes when kids share 'their' beliefs it's ok to say things like, wow I never heard of that before (honestly, sometimes they come up with some of the funniest stuff) or I've never practiced that, tell me about it. The library should be a place and the librarian a person to encourage sharing our beliefs, ideas, backgrounds, and traditions. It promotes lifelong learning and sharing and discourages judgmentalism. Kids seem to want some kind of validation and letting them know that their opinion is just that - their opinion and that other people may believe differently is ok. Different doesn't mean wrong, different is just different. Ok, off my soap box now. :)