Who are you and what did you do with the foa that has a decent grasp of constitutional law? The First Amendment applies to local ordinances as well, through the due process clause of the 14th Amendment. You're right that businesses can still sell the shirts, but it's a legislative act that determines what content can and cannot be displayed and sold to minors. That's clearly unconstitutional based on Brown v. EMA, which relatively recently struck down a California law that banned the sale of violent video games to minors. That decision also cited older SCOTUS cases that struck down laws that, among others, tried to regulate the display of any nudity in drive-in theaters that could potentially be seen by minors driving by. Unless it fits into an obscenity exception, state and local governments do not have the power to regulate the exchange of speech solely based on the content of the message (which it is doing here).
I'm sure you're aware of this case, but for the edification of others I'll also cite Cohen v. California.
Fighting words generally applies to speech directly targeted at specific people. I suppose it could be argued that wearing such a shirt to a NAACP rally would qualify, but the fighting words doctrine has been applied less and less over the years. It's a stupid and vague exception anyway (albeit not as stupid and vague as obscenity), and deserves to be on the shelf.