It's the difference between "it's a crime to do something stupid" and "it's a crime to do concrete thing x, no matter what you're thinking when you do it."
You get deterrence with the second type because then any time a person gives medicine to a child, they think "Hey, I'm breaking the law."
I don't agree. It isn't the law that provides a deterrent effect, it is the enforcement. People in the US aren't terribly averse to breaking the law but are rather more concerned with the likelihood and severity of potential consequences.
Monkeyleg has a good example: drunk driving. They never think they endanger people - that's why the crime isn't "to drive dangerously while drunk", it's driving with a set amount of alcohol that makes the crime, negligence/intentions aside. So now you have people thinking twice before they drink and drive, even if they don't believe they'd be dangerous on the road.
Drunk driving has been illegal in many states for nearly 100 years - much of that time with little impact. It has only been since targeted enforcement and increased penalties took hold in the 70's and 80's that the percentages of fatalities have started falling. People tend to believe they won't get caught violating a law just as strongly as they believe they aren't endangering others by their activities. Thus, your idea of passing a law to prohibit all administration of all drugs to children without a license in order to provide a deterrent effect would fail utterly unless and until it was accompanied by a well-publicized program of testing kids for all potential drugs (including aspirin and acetaminophen) and severely punishing the parents who broke the law. Of course, an equally funded PR campaign advertising the dangers of giving narcotics to babies - both to the child at the time and to the parent after the fact - would be similarly deterrent without all the attendant garbage of yet another piece of unenforceable legislation.
