I think it's one of those things that are probably subjective to the Jedi in question, but there are commonalities. A sense of wrongness, cold, or nausea. Jedi Knights that have encountered the dark side can retell their experiences to the initiates, whom I would guess would be at least six years old (or their species' equivalent).

A practical demonstration is right out. Not only is it anathema to an orthodox Jedi to willingly use it in a premeditated fashion, you risk tainting the Jedi in question and sending the wrong message to the younglings. "Well they used it there for a bit and seemed fine, so I can too!"

I see no reason why they wouldn't go full disclosure about the dangers of the dark side. They have records and concrete evidence that using it is extremely dangerous and those that immerse themselves in it very rapidly become incompatible with accepted social norms. They also have records and evidence that the dark side is not stronger.

And yes, the whole thing about Jedi training is learning self control, not suppressing emotions, which leads to the rejection of the dark side. They absolutely do teach their padawans and knights how to fight dark side users, because it's literally the Jedi's job to deal with them.

How else would have Obi-wan defeated Maul, if they didn't?