When they are old enough to understand, watch with them, and gently explain the horror to them, and be ready to turn the TV off if it is too much for them. :P
it might make a fitting punishment if they ever do something cruel and evil
At the point when you're explaining that sadly, there are intolerant and bigoted people in the world. You can use Faux News as a visual aid, if the child can stomach it.
About 18 years before you let them watch liberal democrats.