I only said that in some cases making a law might not be worth it, not all cases. There are many instances where I would advocate for making a law against an action even though it is rare. An action that results in harm towards a child would almost certainly be one of those instances.
Let me give an example of what I was thinking of. Say that somebody committed some sort of not-yet-illegal fraud that resulted in a loss of taxpayer money. But say that this type of fraud has only happened once, is unlikely to happen often in the future, and in order to enforce any law banning that type of fraud, we’d need to establish new types of police forces, new training methods for the police, new investigative techniques, etc. Establishing this new infrastructure would likely cost the taxpayers far more than what they lose as a result of this very rare form of fraud. So, making this type of fraud illegal might not be worth it, because it would be more expensive to the taxpayer to enforce the new law than it would be to ignore it, and the entire reason why the fraud was wrong/evil in the first place was because it wasted taxpayer dollars.
Obviously there’s no applying a dollar value to a human life, so my thought does not apply to violent behavior. But you were asking about laws in general, not just laws against violent behavior.
My point being, you were making a universal statement that there is never a reason not to outlaw something wrong/evil just because it is rare, and I am arguing that in some cases there is.