They have a point.
Honestly, I did enjoy reading about the different cultures and beliefs. It is actually good to know what others believe and why, so you can hold a decent conversation with them.
I would make certain just "what" is taught. Teaching the facts is one thing, however, if any one "faith" is being pushed on the kids as being the "right"one, that would be a no no.
My only issue is that usually these classes are not all-inclusive and often completely step over anything Christian.
My sister's kids were taught about different cultures starting in 3rd grade!!!! Yes, that early. They came home talking all about Jewish Hanukkah, Islamic Eid and Indian Dewali.....and yet, nothing about Christian Christmas or Easter. Nothing.
The teachings are usually one sided....and therefore, the impetus is on us, to ensure that our children "know" their own Faith and are deeply rooted and strong within it. The schools won't do it, nor, in fact should they. It's our responsibility that the kids learn about and grow to love Orthodoxy.