Interesting issue. I read how Google purged a bunch of apps from the Play Store last year for being able to track you by listening to ultrasonic beacons, but the phone is listening for those sounds, not generating them. I wonder if there are apps that are actually generating ultrasonic tones, and these beacons can also listen for them.
https://www.extremetech.com/mobile/2...trasonic-waves
The next time you're around your kids, boot your phone into Safe Mode:
https://support.t-mobile.com/docs/DOC-28008. This will temporarily disable all 3rd party apps. If the tones go away, then you know it's something you installed (but unfortunately, there's no easy way to narrow it down further, except for uninstalling apps one by one). If the tones persist, then something in the system is causing it.
Or maybe someone installed this as a joke (or not!):
Teenager Repellent: Ultrasound 