In practice parents and the government agree most of the time because it's established science that no one contests, and cases where the government got it wrong are a large class of exceptions. Then people see the government forcing its rules even when they're in the wrong and lose faith in the whole system, including the general consensus. Then you get more counter-culture types fooling around with weird plants.
Whereas in the absence of coercion, the people who are right when the government is wrong are able to openly prove it without being punished, which allows the government to improve its recommendations more rapidly, which reduces how often their recommendations are wrong and thereby the number of people disregarding them wholesale after watching them be incompetent pigheaded bullies.