I thinks it's great and necessary that they learn about varied religions and I'm glad they were exposed to it. My objection is that there is no way the school district would let me go in and explain that Christmas is not Santa and gifts but a message of peace, hope, and joy.
Our school (public) is very open to having kids exposed to all religions, if it is done, as required by law, in a way that is educational and not promotional. We do have parents talk about Eid and Hannukah, but we also invite parents in to talk about Christmas. I would definitely encourage you to talk to your school and your PTA about ways to include all religions, backgrounds, beliefs... Honestly, though, I figure my kids hear all they could possibly need to hear about Christmas (we're practicing Christians) from us and through our society and so I actually love when they hear other things at school. Sometimes, schools just need a reminder about what is okay . This is a great publication to start a conversation: http://www.freedomforum.org/publications/first/teachersguide/teachersguide.pdf
I will also say that we're fairly left-of-center Christians (I cringe, cringe, cringe when I hear about summer vacations to the Creation Museum or talk to someone who dismisses evolution) so maybe that's why, but when I hear Christians in our school district say they feel persectuted or "left out," it's hard to take it seriously (I am NOT saying that is how you feel). After all, as one parent said when we discussed it in a PTA forum, "my kids don't get off for Eid, but your kids get off for Christmas.... who should feel marginalized?"