How is not banning / removing / restricting a book "enforc[ing] one group’s values on the children"? Just having a book available for those kids who want it is not the same as forcing all kids to read it.
The sentence you quoted literally presumes that that is the case. I say if you think it’s ok for one group to impose its values on kids as a whole you should think what these parents are doing (trying to impose their values) is ok.
But I suggest we should instead try to look for non-zero-sum solutions where neither group does that.
IMO the non-zero-sum solution is exactly where neither group imposes their values on the kids, i.e. making the book available for those who want to read it (which will have to mean available for all of them, since we can't know in advance which of them will want to), without either making it mandatory to read or forbidding them from reading it.
Seems to me one of the sides of this issue is a lot closer to that ideal than the other.