1. Parents shouldn't dictate what their students are taught about--or taught. School teachers and administrators are professionals. They know more than Clem and Wilma Cornpone about teaching. It's odd, because almost no one tries to intervene in, say, their child's medical care and tell the doctor what medications he should be prescribing.
2. "Black history" is a subgenre of "history," just like "American history" or "the history of transportation." The reason why black history exists as a subgenre is that the contributions of black people to American society have historically been overlooked. I personally have learned about dozens of highly significant events and individuals that had they involved white people, would have had their own chapters in American history books.
3. If white parents don't want their children to learn about black history, I think that at a minimum, they should be required to come up with good-faith, common-sense objections. I'm not buying any arguments that it's somehow unimportant or tangential. To cite a demographic that's roughly comparable in significance, I doubt that anyone would object to, say, a course on Scandinavian-American history. So this smells like racism, and I doubt anyone can justify it without twisting themselves into moral and legal knots.
4. And yeah, I think we as a nation need to confront and acknowledge our sometimes awful history, especially since its repercussions are still being felt today: Electoral College. Income inequality. Gerrymandering. Voting rights. Good ol' fashioned American racial discrimination and bias. The demonizing of immigrants. Et cetera. Et cetera. Et cetera. Ignoring or whitewashing it won't make it go away.