There were no warning leaflets dropped on these cities warning of the atomic bomb attacks. At least, not until after the attacks (Nagasaki got warning leaflets the day after it was nuked). The notion that they were so warned is a common but utterly confused and completely debunked misconception. More details here.

They may have gotten generic "we will be bombing cities" leaflets but even this seems unproven. We have many samples of wartime warning leaflets (e.g. LeMay leaflets) and I have never seen one that mentioned these cities as targets. (And I have looked!)

But to your general point: Hiroshima had conspicuously not been bombed and people were indeed afraid that their number was coming up. A large group of evacuees were in fact gathering in the center of town on the morning of August 6th, when the bombing happened. Bad timing. (Nagasaki by contrast had been conventionally bombed previously.)

Many people in Japan did evacuate to the countryside, or send their children there. But there were diminishing food supplies there. One cannot just displace millions of people from all major Japanese cities into the countryside -- it wouldn't work today and it wouldn't work then. The US leaflet campaigns were never about the idea that the populations would be saved. They were psychological warfare campaigns designed to hurt Japanese morale and degrade the Japanese industrial workforce.

In many cases of leaflets dropped warning of impending air raids, Japanese civilians were warned by their own authorities not to read the leaflets or risk being subjected to a fine and prison time. The Japanese government did this because they didn't want to lose morale and also lose civilians who were employed in manufacturing munitions needed to keep up with the war. In spite of this, however, most of the Japanese civilians did evacuate the cities to the countryside, most of them who happened to be kids. However, the countryside was sufficiently lacking of food and it takes a lot of logistics and personnel to oversee the evacuation in the fashionable manner. So either the Japanese civilians stayed in cities or were forced to stay in the cities in order to support the war effort.

There was no warning about the atomic attacks, which was useless to the Japanese people anyway if we did give them a warning. The idea of having a single bomb destroying an entire city with a blink of an eye was alien and was something straight out of a science fiction book. For months prior to the use of nukes in combat, hundreds of B-29s were dropping 1,000 tons of conventional firebombs on Japanese cities, gutting most of the areas and killing hundreds of thousands of people in the process, which can take hours or months. The world in pre-August 6, 1945, knew what bombs can do, but not the idea of having a single bomb destroying an entire city with a blink of an eye. So nobody in Hiroshima and Nagasaki were aware of single B-29s delivering single atomic bombs and vaporizing them in seconds. Often times, the leaflets would be dropped on the cities, including Hiroshima and Nagasaki, about the incoming fire raids, not the atomic raids.