I am not flaired, but I am a professional working comic artist, with both a bachelors and masters in art and a minor in art history. Just to establish credentials...

The thing to understand is that in those days, the distribution system of comics was completely different. Comics were printed in the millions and sent to newstands and drug stores along with magazines and newspapers, often by the same distribution companies. This system was highly regional and had hundreds of competing companies across the country.

Because of this, you did not have to go to a specialty comic shop to get comics; they were available (and only available) at the same places people bought their papers. This means that comics were nearly ubiquitous across the country at newsstands. This was the only way comics could be bought until the rise of the Direct Market and specialty comic shops in the mid-70s.

Its also important that Superman was incredibly popular. He was introduced in 1938 and by the next year had already expanded into newspaper strips and other media and his image was plastered on every item you could imagine. And this was not just trinkets for kids; Superman was selling everything, from milk to gasoline. This is the reason he got his own radio show in the first place!

His introduction also created a giant boom in comics known as the Golden Age. During this time, the circulation numbers for popular heroes was staggering: millions upon millions of copies were sold and read. Much, much higher than the sales figures today.

This boom in comics lasted until the mid-50s, when the US Senate held hearings about gore and violence in comics leading to juvenile delinquency, resulting in the establishment of a self-censorship body, the Comics Code Authority.

Suffice to say that Superman was ridiculously popular nearly at his inception, his image was reproduced on all types of other items, and his comics sold in the millions and were at every newstand and drugstore in the country, every month

I think it is unlikely that a kid in 1940 would be unaware of what superman looked like if he or she were fans of the radio show, even accounting for rural listeners, etc. Superman was truly a cultural phenomenon, right from the beginning.

Firstly, comic books were not the niche content that they are today (superhero movies notwithstanding), they were everywhere and ubiquitous. Today you have to trek out to a comic shop to buy them and you have to shell out several dollars for each issue. This is in many ways a response to their past history of ubiquity and popularity (which created a cycle of nostalgia and collectability which massively inflated prices and changed the market during the '80s and '90s, but that's another topic). Back in the "Golden Age" of comics in the 1940s they were dirt cheap and they were everywhere.

Anywhere you could find magazines or newspapers you would likely also find comic books for sale. Grocery stores, convenience stores, kiosks (newsstands), etc. And they would cost typically 5 or 10 cents an issue. They had to be cheap because they were largely being marketed to kids who didn't have a ton of spending cash. But at the same cost of one or two sodas they became affordable, and lots of kids bought them. By the 1940s there were hundreds of different comics in print, and total sales were astronomical. By 1950 both Marvel and DC (among a zillion others) were selling millions of comic books every month.

However, even though very likely an overwhelmingly large portion of the listeners to the Superman radio show owned at least one comic none of that was necessary because almost every household was getting Superman comics daily in the form of the comic strip printed in the newspaper. These ran from 1939 up until the mid-1960s, with a larger color strip added for Sundays later on in 1939. When The Adventures of Superman radio show started up in 1940 essentially every listener was already familiar with Superman from the comic strip. This pattern played out with a great many other comic strips as well in the same time frame: Red Ryder, Dick Tracy, Little Orphan Annie, Skippy, Mark Trail, etc.

Just like today the 1940s was awash in disposable media, it wasn't a rare or precious thing the way we might imagine it would have to be by plotting a reverse trajectory from today's ubiquity and chaos back to a seemingly simpler and more austere age. But no, they too were sitting on mountains of media. Comic books of near infinite variety readily available everywhere for a pittance, comic strips every day in the paper you were already getting, and serials on the radio and in the theaters.