D”Y, holding hands with a niddah is NOT the same issur as eating at McDonald’s, and anyone who says so, is probably committing the same type of lo tosif as Adam Harishon did when he told Chava they could not even touch the eitz hadaas, whent hat was NOT what Hashem said to Adam.
I am not saying that anyone SHOULD touch a niddah, but if they do, lightning will most likely not strike them, and they will therefore lose their desire to refrain from doing so. It is far better IMO to tell young people that the reasons for shemiras negiya are specifically to help them to avoid a situation that can strongly lead to an aveira that IS chayav koreis. But they should not be told that they would get koreis for touching a niddah, if it simply is not true. Then they don’t believe that which IS true.