It's always important not to wait until your child comes home to find out what the public school system is pumping into them. It is vital that you are knee deep in curriculum. Go to the teacher and request a copy of the curriculum. They have to provide you with a copy. That way, when you ask your child when they come home "what did you learn in school today" you already know the subject matter and are prepared to discuss. (I hate surprise topics.)
Also, it's important to monitor textbooks your child is using. I was appalled at some of the crap in my 5th grader's history textbook and my 10th grader's civics book. Many textbooks are available online, some in PDF format. This means my 10th grader doesn't have to lug 50 pounds of books home in his backpack (I'm not kidding.) If you find a mistake, you can correct with your child (I use post it notes, since writing in the book itself means you bought the book), and if you find something about how the KKK was an organization made up of righteous people who were interested in "securing the rights of whites over inferior negros" (I'm not kidding) then bring it up to the teacher and follow up with the school board, who are the people that select textbooks. Make sure the offending portion is removed or they use a different publisher.
The key is to get involved, and not wait for surprise questions.
