I was taught pretty offensive things about protestants at school. Even with the low political sophistication of a 10yo I could spot crap when it was being spouted.
A problem with censoring "bad" ideas is that we don't help kids deal with subtle issues.
Holocaust denial is a good case in point. It is instructive to listen to their arguments which are consistent in ideas if not specifics.
David Irving being an arts grad simply can't do numbers, and thus has asserted that the Germans simply could not have moved that many Jews, and that the camps couldn't have handled them anayway.
This is risible bollocks. A well educated 10yo should be able to kick holes in that.
Also they rarely actually deny the existence of the holocaust, merely saying it's "exaggerated". There they are on safer ground since their has been a sad degree of competitive victimhood between those who claim to speak for Jews, Gypsies, Gays et al, and it's not too hard to suspect the precise numbers.
Also they have intersected with mainstream Christian group who have long cliamed that we bombed the fuckers too much in Dresden, Hiroshima et al. Thus Germans have managed to weasel their way into nice fuzzy victimhood.
But their argument implies that there is some acceptable number of Jews et al that they could have gassed so that it was OK. Typically theses claims saythe exaggeration is anywhere between 50 and 500%, implying that that if they'd only killed 1 or 2 million that would have been right and proper.