The only people that can and should decide when their child is ready for that material is their parents.

True, and I'm fine with a parent going to the teacher and saying "I don't think my kid is ready for this and would prefer an alternative assignment; can we work something out here?"

And it's not even a case of 'well even if they don't read it in class, they can find it elsewhere'-- I would've loved the chance to get a group input on some of the books I was reading in high school; for me that chance came in English class.

Sorry, but I'll just never see the solution to this problem being 'take away the opportunity for discussion from the whole class because some of their parents don't like the material'.