I'm no expert on Holocause denial, but I know its proponents cite 'evidence' of their own, and have arguments to rebut the conventional viewpoint. Presumably their eivdence and arguments can easily be dismissed, but I'd rather the child see that for themselves, rather than just being told that it's the case.

Although you can't cover every crackpot theory there is, I think Holocaust denial is one of the more important ones to talk about, from the point of view of its impact on quite large sectors of society.

(Also, note I'm not saying that we should "teach theories that are in fact discredited", but that we should "teach about theories that are in fact discredited".)