Yes – you are right. Atmospheric (pressure) waves of this frequency can be heard. I by myself me to hear 19KHz stereo indicator and *was able* to hear it.
Transmission of SAQ is *NOT* a transversal pressure wave but an electromagnetic wave. This wave can cross vaccuum, too – sound waves need medium ”air” to spread themselves….
So a child cannot hear anything of SAQ transmission – because living people do not have a sensor for electromagnetic waves (like an ear for sound waves) 🙂
73 (best regards) de