You could come up with corner cases and odd delineations for those former examples too. Yet we all know the gist of the laws and manage to somehow, on the aggregate, prevent kids from engaging in said activity.

E.g. alcohol. How do we stop kids form drinking alcohol? How do you define alcohol? What about kids medicine that includes alcohol, what about medical procedures that insert substances with alcohol content. What about shops that get conned by kids with fake ID. What if the label gets scratched off and kid doesn't know it had alcohol. What if a parent gives their child a sip of beer. What about old home remedies that include whiskey. What about colic medicine that has alcohol, okay What about carving exceptions for babies, etc etc etc.

Granted some of those are contrived, but it's not as black and White as you think.